Add docstring to cmd.Cmd.do_help()

Doc/library/cmd.rst

@@ -312,10 +312,6 @@ blank lines to repeat commands, and the simple record and playback facility::
     bye     color    goto     home  playback  record  right
     circle  forward  heading  left  position  reset   undo
 
-    Undocumented commands:
-    ======================
-    help
-
     (turtle) help forward
     Move the turtle forward by the specified distance:  FORWARD 10
     (turtle) record spiral.cmd

Lib/cmd.py

@@ -288,6 +288,7 @@ class Cmd:
         return list(commands | topics)
 
     def do_help(self, arg):
+        'List available commands with "help" or detailed help with "help cmd".'
         if arg:
             # XXX check arg syntax
             try:

Lib/test/test_cmd.py

@@ -84,11 +84,11 @@ class samplecmdclass(cmd.Cmd):
     <BLANKLINE>
     Documented commands (type help <topic>):
     ========================================
-    add
+    add  help
     <BLANKLINE>
     Undocumented commands:
     ======================
-    exit  help  shell
+    exit  shell
     <BLANKLINE>
 
     Test for the function print_topics():
@@ -125,11 +125,11 @@ class samplecmdclass(cmd.Cmd):
     <BLANKLINE>
     Documented commands (type help <topic>):
     ========================================
-    add
+    add  help
     <BLANKLINE>
     Undocumented commands:
     ======================
-    exit  help  shell
+    exit  shell
     <BLANKLINE>
     help text for add
     Hello from postloop