make it absolutely clear that Purge History API does not remove all traces of...

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

docs/admin_api/purge_history_api.rst

@@ -4,6 +4,8 @@ Purge History API
 The purge history API allows server admins to purge historic events from their
 database, reclaiming disk space.
 
+**NB!** This will not delete local events (locally sent messages content etc) from the database, but will remove lots of the metadata about them and does dramatically reduce the on disk space usage
+
 Depending on the amount of history being purged a call to the API may take
 several minutes or longer. During this period users will not be able to
 paginate further back in the room from the point being purged from.