Signed-off-by: Travis Ralston <travpc@gmail.com>

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.

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