finagle/doc: Update Offload Admission Control documentation

Problem:

Update the Offload Admission Control documentation to reflect current state.

Solution:

Remove the experimental warning.

JIRA Issues: CSL-11928

Differential Revision: https://phabricator.twitter.biz/D873537

doc/src/sphinx/ThreadingModel.rst

@@ -156,8 +156,6 @@ Or to enable  with a manually tuned threading configuration:
 Offload Admission Control
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-.. warning:: This is an experimental feature and may undergo rapid developmental changes.
-
 .. note:: This AC mechanism can only work if global offloading is enabled.
 
 Using system-wide offloading opens the door to an experimental form of admission control. This