Why is M-; not showing up in <lang-mode> C-h m help?

[Sivaram Neelakantan]

I just noticed that M-; (comment-dwim) does not show in C-h m for the
language/source code one is writing the script for.  Why is that?  In
sh-mode,python mode it's not there at all.  Given that
commenting/uncommenting is fairly common task in source editing,
shouldn't the key map be part of the mode help too?  

It does show up under C-h k but that's irrelevant to a new user of a
language-mode.

I only noticed this now because of a post in comp.Emacs from someone
asking for help on commenting.

 sivaram
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