[3.13] docs: puremagic.what() as replacement for imghdr.what() (GH-120871) (#120951)

docs: puremagic.what() as replacement for imghdr.what() (GH-120871)
(cherry picked from commit 22b8a35)

Co-authored-by: Christian Clauss <cclauss@me.com>

Author: miss-islington

Doc/whatsnew/3.13.rst

@@ -1295,6 +1295,9 @@ PEP 594: dead batteries (and other module removals)
 
   * :mod:`!imghdr`: use the projects :pypi:`filetype`,
     :pypi:`puremagic`, or :pypi:`python-magic` instead.
+    The ``puremagic.what()`` function can be used to replace
+    the ``imghdr.what()`` function for all file formats that
+    were supported by ``imghdr``.
     (Contributed by Victor Stinner in :gh:`104773`.)
 
   * :mod:`!mailcap`.