bug#6616: S-TAB is mismapped in the *Help* buffer

[Drew Adams]

> > and binding [S-tab] to [backtab] in the 
> `ns-alternatives-map' instead?
> 
> Actually, we might want to do that everywhere, rather than only in
> x-win.el and ns-win.el.

I disagree.  S-TAB has always been a free global key, and it has remained free
in most keymaps.

In Emacs there are several different behaviors that here and there are
associated with TAB.  The use of TAB for navigation in the sense of being
opposite to [backtab] (e.g. navigation in Info or *Help*) is only one of them,
and it is a fairly minor one (for Emacs).  We only recently added it to Info.
Arguably it can be said to make sense for other, similar read-only modes such as
*Help*.  Beyond that it does not necessarily make sense.

Please do not bind S-TAB in a general way to [backtab] or anything else.  Modes
that really need that can do so.  That leaves other code (e.g. other modes, user
code, 3rd-party code) free to use S-TAB for other uses, especially uses that are
related to a particular use of TAB.  TAB for navigation is only one use of TAB.

We already have potential and some real conflicts between different meanings of
TAB.  A mode needs to choose which meaning it prefers when there is a potential
conflict.  Some modes try to combine such behaviors into a DWIM behavior.  This
is enough - let's not make this more problematic by throwing S-TAB into the mix
in a predefined way.  Let the code in question decide.

Bind S-TAB to `backtab' for *Help* if you want - that's helpful.  Likewise
perhaps some view-mode contexts (maybe all of them; dunno).  But otherwise
please leave it alone.  Thx.