Docs: mature TCP keep alive (#967)

This config option has existed for many versions. It's stable in other drivers
as well. In fact, the driver didn't even issue a warning on using it. It was
purely marked as experimental in the API docs. This PR aims to change this by
removing the experimental mark in the docs.

Author: robsdedude

docs/source/api.rst

@@ -454,9 +454,6 @@ Specify whether TCP keep-alive should be enabled.
 :Type: ``bool``
 :Default: ``True``
 
-**This is experimental** (see :ref:`filter-warnings-ref`).
-It might be changed or removed any time even without prior notice.
-
 
 .. _max-connection-lifetime-ref: