Spell out that the last event sent to a room won't be deleted by a purge

changelog.d/6891.doc

@@ -0,0 +1 @@
+Spell out that the last event sent to a room won't be deleted by a purge. 

docs/admin_api/purge_history_api.rst

@@ -8,6 +8,9 @@ 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.
 
+Note that Synapse requires at least one message in each room, so it will never
+delete the last message in a room.
+
 The API is:
 
 ``POST /_synapse/admin/v1/purge_history/<room_id>[/<event_id>]``

docs/message_retention_policies.md

@@ -42,6 +42,10 @@ purged according to its room's policy, then the receiving server will
 process and store that event until it's picked up by the next purge job,
 though it will always hide it from clients.
 
+Synapse requires at least one message in each room, so it will never
+delete the last message in a room. It will, however, hide it from
+clients.
+
 
 ## Server configuration