Update the coding style doc (#5719)

A few fixes and removal of duplicated stuff, but mostly a bunch of the words on the config file.

changelog.d/5719.misc

+Update the coding style document.

docs/code_style.rst

-# Code Style
+Code Style
+==========
+
+Formatting tools
+----------------
 
 The Synapse codebase uses a number of code formatting tools in order to
 quickly and automatically check for formatting (and sometimes logical) errors
@@ -6,20 +10,20 @@ in code.
 
 The necessary tools are detailed below.
 
-## Formatting tools
+- **black**
 
-The Synapse codebase uses [black](https://pypi.org/project/black/) as an
-opinionated code formatter, ensuring all comitted code is properly
-formatted.
+  The Synapse codebase uses `black <https://pypi.org/project/black/>`_ as an
+  opinionated code formatter, ensuring all comitted code is properly
+  formatted.
 
-First install ``black`` with::
+  First install ``black`` with::
 
-  pip install --upgrade black
+    pip install --upgrade black
 
-Have ``black`` auto-format your code (it shouldn't change any
-functionality) with::
+  Have ``black`` auto-format your code (it shouldn't change any functionality)
+  with::
 
-  black . --exclude="\.tox|build|env"
+    black . --exclude="\.tox|build|env"
 
 - **flake8**
 
@@ -54,17 +58,16 @@ functionality is supported in your editor for a more convenient development
 workflow. It is not, however, recommended to run ``flake8`` on save as it
 takes a while and is very resource intensive.
 
-## General rules
+General rules
+-------------
 
 - **Naming**:
 
   - Use camel case for class and type names
   - Use underscores for functions and variables.
 
-- Use double quotes ``"foo"`` rather than single quotes ``'foo'``.
-
-- **Comments**: should follow the `google code style
-  <http://google.github.io/styleguide/pyguide.html?showone=Comments#Comments>`_.
+- **Docstrings**: should follow the `google code style
+  <https://google.github.io/styleguide/pyguide.html#38-comments-and-docstrings>`_.
   This is so that we can generate documentation with `sphinx
   <http://sphinxcontrib-napoleon.readthedocs.org/en/latest/>`_. See the
   `examples
@@ -73,6 +76,8 @@ takes a while and is very resource intensive.
 
 - **Imports**:
 
+  - Imports should be sorted by ``isort`` as described above.
+
   - Prefer to import classes and functions rather than packages or modules.
 
     Example::
@@ -92,25 +97,84 @@ takes a while and is very resource intensive.
     This goes against the advice in the Google style guide, but it means that
     errors in the name are caught early (at import time).
 
-  - Multiple imports from the same package can be combined onto one line::
+  - Avoid wildcard imports (``from synapse.types import *``) and relative
+    imports (``from .types import UserID``).
 
-      from synapse.types import GroupID, RoomID, UserID
+Configuration file format
+-------------------------
 
-    An effort should be made to keep the individual imports in alphabetical
-    order.
+The `sample configuration file <./sample_config.yaml>`_ acts as a reference to
+Synapse's configuration options for server administrators. Remember that many
+readers will be unfamiliar with YAML and server administration in general, so
+that it is important that the file be as easy to understand as possible, which
+includes following a consistent format.
 
-    If the list becomes long, wrap it with parentheses and split it over
-    multiple lines.
+Some guidelines follow:
 
-  - As per `PEP-8 <https://www.python.org/dev/peps/pep-0008/#imports>`_,
-    imports should be grouped in the following order, with a blank line between
-    each group:
+* Sections should be separated with a heading consisting of a single line
+  prefixed and suffixed with ``##``. There should be **two** blank lines
+  before the section header, and **one** after.
 
-    1. standard library imports
-    2. related third party imports
-    3. local application/library specific imports
+* Each option should be listed in the file with the following format:
 
-  - Imports within each group should be sorted alphabetically by module name.
+  * A comment describing the setting. Each line of this comment should be
+    prefixed with a hash (``#``) and a space.
 
-  - Avoid wildcard imports (``from synapse.types import *``) and relative
-    imports (``from .types import UserID``).
+    The comment should describe the default behaviour (ie, what happens if
+    the setting is omitted), as well as what the effect will be if the
+    setting is changed.
+
+    Often, the comment end with something like "uncomment the
+    following to \<do action>".
+
+  * A line consisting of only ``#``.
+
+  * A commented-out example setting, prefixed with only ``#``.
+
+    For boolean (on/off) options, convention is that this example should be
+    the *opposite* to the default (so the comment will end with "Uncomment
+    the following to enable [or disable] \<feature\>." For other options,
+    the example should give some non-default value which is likely to be
+    useful to the reader.
+
+* There should be a blank line between each option.
+
+* Where several settings are grouped into a single dict, *avoid* the
+  convention where the whole block is commented out, resulting in comment
+  lines starting ``# #``, as this is hard to read and confusing to
+  edit. Instead, leave the top-level config option uncommented, and follow
+  the conventions above for sub-options. Ensure that your code correctly
+  handles the top-level option being set to ``None`` (as it will be if no
+  sub-options are enabled).
+
+* Lines should be wrapped at 80 characters.
+
+Example::
+
+    ## Frobnication ##
+
+    # The frobnicator will ensure that all requests are fully frobnicated.
+    # To enable it, uncomment the following.
+    #
+    #frobnicator_enabled: true
+
+    # By default, the frobnicator will frobnicate with the default frobber.
+    # The following will make it use an alternative frobber.
+    #
+    #frobincator_frobber: special_frobber
+
+    # Settings for the frobber
+    #
+    frobber:
+       # frobbing speed. Defaults to 1.
+       #
+       #speed: 10
+
+       # frobbing distance. Defaults to 1000.
+       #
+       #distance: 100
+
+Note that the sample configuration is generated from the synapse code and is
+maintained by a script, ``scripts-dev/generate_sample_config``. Making sure
+that the output from this script matches the desired format is left as an
+exercise for the reader!