Reword the sample config header to be less scary (#4801)

changelog.d/4801.feature

+Include a default configuration file in the 'docs' directory.

docs/.sample_config_header.yaml

-# This file is a reference to the configuration options which can be set in
-# homeserver.yaml.
+# The config is maintained as an up-to-date snapshot of the default
+# homeserver.yaml configuration generated by Synapse.
 #
-# Note that it is not quite ready to be used as-is. If you are starting from
-# scratch, it is easier to generate the config files following the instructions
-# in INSTALL.md.
+# It is intended to act as a reference for the default configuration,
+# helping admins keep track of new options and other changes, and compare
+# their configs with the current default.  As such, many of the actual
+# config values shown are placeholders.
+#
+# It is *not* intended to be copied and used as the basis for a real
+# homeserver.yaml. Instead, if you are starting from scratch, please generate
+# a fresh config using Synapse by following the instructions in INSTALL.md.
 

docs/sample_config.yaml

-# This file is a reference to the configuration options which can be set in
-# homeserver.yaml.
+# The config is maintained as an up-to-date snapshot of the default
+# homeserver.yaml configuration generated by Synapse.
 #
-# Note that it is not quite ready to be used as-is. If you are starting from
-# scratch, it is easier to generate the config files following the instructions
-# in INSTALL.md.
+# It is intended to act as a reference for the default configuration,
+# helping admins keep track of new options and other changes, and compare
+# their configs with the current default.  As such, many of the actual
+# config values shown are placeholders.
+#
+# It is *not* intended to be copied and used as the basis for a real
+# homeserver.yaml. Instead, if you are starting from scratch, please generate
+# a fresh config using Synapse by following the instructions in INSTALL.md.
 
 ## Server ##