pythongh-72570: mention the incompatibility of XOFs with HMAC (python#136676)

Author: picnixz

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)