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

[Drew Adams]

> > I tried looking for the place in the Emacs manual where the notation
> > for key sequences (the one used in the doc) is described. I tried
> > `notation' and found there was only one match:
> > `gdb-use-colon-colon-notation'. Users should be able to find this by
> > looking for terms such as `notation' and `conventions'.
> 
> In what circumstances does Emacs talk about notations for key 
> sequences? 

The conventions used in the doc should be described.  They are, for the Elisp
manual: see node `Conventions' and its subnodes.  See how detailed those nodes
are, and how well they explain the notations used in the Elisp manual.

The Emacs manual should also describe the notations it uses.  In particular,
key-sequence notation.

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).

If there is not yet such a desciption, it should be added.  And then it should
be indexed (this bug report).