bpo-36794: Document that Lock.acquire is fair. (GH-13082)

https://bugs.python.org/issue36794

Author: hniksic

Doc/library/asyncio-sync.rst

@@ -66,6 +66,13 @@ Lock
       This method waits until the lock is *unlocked*, sets it to
       *locked* and returns ``True``.
 
+      When more than one coroutine is blocked in :meth:`acquire`
+      waiting for the lock to be unlocked, only one coroutine
+      eventually proceeds.
+
+      Acquiring a lock is *fair*: the coroutine that proceeds will be
+      the first coroutine that started waiting on the lock.
+
    .. method:: release()
 
       Release the lock.