[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 22b8a35d6e6660cf7457ed6636cb8c12fff7e8e7)

Co-authored-by: Christian Clauss <cclauss@me.com>
diff --git a/Doc/whatsnew/3.13.rst b/Doc/whatsnew/3.13.rst
index 35853ff..971f92c 100644
--- a/Doc/whatsnew/3.13.rst
+++ b/Doc/whatsnew/3.13.rst
@@ -1295,6 +1295,9 @@
 
   * :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`.