clarify that `purge_history` will not purge state

Make the purge request return quickly, and allow scripts to poll for updates.

* It's supposed to be purge_local_events, not ..._history
* Fix the doc to have valid json

Add a flag which makes the purger delete local events

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.