bpo-36277: Add document for pdb debug and retval commands (GH-12872)

https://bugs.python.org/issue36277

Automerge-Triggered-By: @csabella
(cherry picked from commit 9391f6c3ef)

Co-authored-by: Dave Nguyen <dv@dvnguyen.com>

Doc/library/pdb.rst

@@ -531,6 +531,14 @@ by the local file.
 
    Quit from the debugger.  The program being executed is aborted.
 
+.. pdbcommand:: debug code
+
+   Enter a recursive debugger that steps through the code
+   argument (which is an arbitrary expression or statement to be
+   executed in the current environment).
+
+.. pdbcommand:: retval
+   Print the return value for the last return of a function.
 
 .. rubric:: Footnotes