Clearly say that the server/server API is unstable and subject to change
Federation API
==============
+.. WARNING::
+ This API is unstable and will change without warning or discussion while
+ we work towards a r0 release (scheduled for August 2018).
+
Matrix homeservers use the Federation APIs (also known as server-server APIs)
to communicate with each other. Homeservers use these APIs to push messages to
each other in real-time, to
