docs: puremagic.what() as replacement for imghdr.what() (python#120871)

Author: cclauss

Doc/whatsnew/3.13.rst

@@ -1313,6 +1313,9 @@ 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`.