Answer "what are redactions?"

supporting-docs/guides/2015-08-19-faq.md

@@ -478,6 +478,10 @@ The precise architecture of identity servers is currently in flux and subject to
 
 Each homeserver stores the communication history and account information for all of its clients, and shares data with the wider Matrix ecosystem by synchronising communication history with other homeservers and their clients. Clients typically communicate with each other by emitting events in the context of a virtual room. Room data is replicated across all of the homeservers *whose users are participating in a given room*.
 
+##### What are redactions?
+
+Since events are extensible it is possible for malicious users and/or servers to add keys that are, for example offensive or illegal. Since some events cannot be simply deleted (e.g. membership events) we instead 'redact' events, essentially stripping the event of all keys that are not required by the protocol. Redacting an event cannot be undone, allowing server owners to also delete the offending content from the databases. 
+
 ##### How do you do VoIP calls on Matrix?
 
 Voice (and video) over Matrix uses the WebRTC 1.0 standard to transfer call media (i.e. the actual voice and video traffic).  Matrix is used to signal the establishment and termination of the call by sending call events, like any other event.  Currently calls are only supported in rooms with exactly two participants - however, one of those participants may be a conferencing bridge.  We're looking at better ways to do group calling.