Remove stability warning
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 retrieve historic messages from each other, and to
