[3.9] bpo-36540: Improve doc of function definition regarding positional-only arguments (GH-25235) (GH-25259)

(cherry picked from commit 58d72ca)

Co-authored-by: Saiyang Gou <[email protected]>

Author: gousaiyang

Doc/reference/compound_stmts.rst

@@ -587,19 +587,25 @@ e.g.::
        return penguin
 
 .. index::
+   single: / (slash); function definition
    single: * (asterisk); function definition
    single: **; function definition
 
 Function call semantics are described in more detail in section :ref:`calls`. A
 function call always assigns values to all parameters mentioned in the parameter
-list, either from position arguments, from keyword arguments, or from default
+list, either from positional arguments, from keyword arguments, or from default
 values.  If the form "``*identifier``" is present, it is initialized to a tuple
 receiving any excess positional parameters, defaulting to the empty tuple.
 If the form "``**identifier``" is present, it is initialized to a new
 ordered mapping receiving any excess keyword arguments, defaulting to a
 new empty mapping of the same type.  Parameters after "``*``" or
 "``*identifier``" are keyword-only parameters and may only be passed
-used keyword arguments.
+by keyword arguments.  Parameters before "``/``" are positional-only parameters
+and may only be passed by positional arguments.
+
+.. versionchanged:: 3.8
+   The ``/`` function parameter syntax may be used to indicate positional-only
+   parameters. See :pep:`570` for details.
 
 .. index::
    pair: function; annotations