[3.14] gh-72570: mention the incompatibility of XOFs with HMAC (GH-136676) (#136678)

gh-72570: mention the incompatibility of XOFs with HMAC (GH-136676)
(cherry picked from commit a02cf19)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: miss-islington

Doc/library/hmac.rst

@@ -12,6 +12,9 @@
 --------------
 
 This module implements the HMAC algorithm as described by :rfc:`2104`.
+The interface allows to use any hash function with a *fixed* digest size.
+In particular, extendable output functions such as SHAKE-128 or SHAKE-256
+cannot be used with HMAC.
 
 
 .. function:: new(key, msg=None, digestmod)