gh-96397: Document that keywords in calls need not be identifiers (GH-96393)

This represents the official SC stance, see
https://github.com/python/steering-council/issues/142GH-issuecomment-1252172695
(cherry picked from commit 9d432b4)

Co-authored-by: Jeff Allen <ja.py@farowl.co.uk>

Author: miss-islington

Doc/reference/expressions.rst

@@ -1049,10 +1049,20 @@ used in the same call, so in practice this confusion does not arise.
 
 If the syntax ``**expression`` appears in the function call, ``expression`` must
 evaluate to a :term:`mapping`, the contents of which are treated as
-additional keyword arguments.  If a keyword is already present
-(as an explicit keyword argument, or from another unpacking),
+additional keyword arguments. If a parameter matching a key has already been
+given a value (by an explicit keyword argument, or from another unpacking),
 a :exc:`TypeError` exception is raised.
 
+When ``**expression`` is used, each key in this mapping must be
+a string.
+Each value from the mapping is assigned to the first formal parameter
+eligible for keyword assignment whose name is equal to the key.
+A key need not be a Python identifier (e.g. ``"max-temp °F"`` is acceptable,
+although it will not match any formal parameter that could be declared).
+If there is no match to a formal parameter
+the key-value pair is collected by the ``**`` parameter, if there is one,
+or if there is not, a :exc:`TypeError` exception is raised.
+
 Formal parameters using the syntax ``*identifier`` or ``**identifier`` cannot be
 used as positional argument slots or as keyword argument names.