bug#3801: 23.0.96; Emacs manual: add index entry for "notation, keys" or "key notation"

[Drew Adams]

> > There should be a description of how the manual indicates 
> > key sequences, including for different kinds of keys (e.g.
> > angle-bracket notation vs no angle brackets).
> 
> I think you are looking for "User Input" and "Keys" nodes.

Yes and no.  Yes, those nodes speak about keys, and they _use_ the notational
conventions defined for the manual.

But no, those nodes do not _present_ (introduce, describe, explain) the
notational conventions that are used.

I don't see any place in the Emacs manual where we introduce the notational
conventions used.  Contrast that with the Elisp manual, where we go into quite a
bit of detail about the notation used in the manual.