Add "rv" short form to retval docstring

Similar to other cases e.g. "ll | longlist"

Author: danra

Lib/pdb.py

@@ -2251,7 +2251,7 @@ def do_args(self, arg):
     do_a = do_args
 
     def do_retval(self, arg):
-        """retval
+        """rv | retval
 
         Print the return value for the last return of a function.
         """