bpo-40773: Fix rendering for 'retval' on the pdb page (GH-21081)

Automerge-Triggered-By: @merwok
(cherry picked from commit cf18c9e)

Co-authored-by: Arisaka97 <solitaire2312@gmail.com>

Author: miss-islington

Doc/library/pdb.rst

@@ -538,6 +538,7 @@ by the local file.
    executed in the current environment).
 
 .. pdbcommand:: retval
+
    Print the return value for the last return of a function.
 
 .. rubric:: Footnotes