There are no known bugs in the message retention code, but
it is possible that there still exists race conditions. Additional
fixes will be made as reported.

* Labeled a lot more code blocks with the appropriate type
* Fixed a couple of minor typos (missing/extraneous commas)

Signed-off-by: Sumner Evans <me@sumnerevans.com>

* Fix broken links in documentation

* newsfile

* Update documentation on retention policies limits

Document the changes from https://github.com/matrix-org/synapse/pull/8104

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>