bpo-46741: Update asyncio.protocols.BufferedProtocol docstring (31327)

The docstring for `BufferedProtocol` states that the class is still an "experimental API", but it has been considered stable since 3.8.
(cherry picked from commit 1d81fdc)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Lib/asyncio/protocols.py

@@ -109,10 +109,6 @@ def eof_received(self):
 class BufferedProtocol(BaseProtocol):
     """Interface for stream protocol with manual buffer control.
 
-    Important: this has been added to asyncio in Python 3.7
-    *on a provisional basis*!  Consider it as an experimental API that
-    might be changed or removed in Python 3.8.
-
     Event methods, such as `create_server` and `create_connection`,
     accept factories that return protocols that implement this interface.