make it absolutely clear that Purge History API does not remove all traces of events and message contents

because this topic pops up too often

#890 #1621 #1730 #2260 #2315 and so on

I haven't (yet) documented all of the user-list APIs introduced in
https://github.com/matrix-org/synapse/pull/1784 because the API shape seems
very odd, given the functionality.

The cache wrappers had a habit of leaking the logcontext into the reactor while
the lookup function was running, and then not restoring it correctly when the
lookup function had completed. It's all the fault of
`preserve_context_over_{fn,deferred}` which are basically a bit broken.

This defines the low level TCP replication protocol

Apparently the github RST renderer doesn't like Note blocks.

It looks like CAPTCHA_SETUP.rst contains information relevant to an old version of Synapse, but Synapse now has a different approach to configuring use of the X-Forwarded-For header.

purge_media_cache takes its arg from a query-param, not the POST body, for some
reason.

this is rst so name it accordingly

Remove long-deprecated instructions about promethesus console; also fix for modern config file format

Not logical to use svn checkout against a github repo, so changed to git clone. 

Signed-off-by: Rick Cogley <rick.cogley@esolia.co.jp>

svn checkout is not logical for a checkout from github, so changed the checkout to "git clone". 
thanks @dbkr

Signed-off-by: Rick Cogley <rick.cogley@esolia.co.jp>