bug#58602: 29.0.50; Please document (:documentation FORM) spec for closures

[Drew Adams <drew.adams@oracle.com>]

> I prefer searching in info via 's', like 's :documentation RET'.  This
> has also the advantage over the 'i' command to show me dark corners of a
> manual which haven't been indexed properly :-)

1. We _can_ have index entries that start with
`:', and we do have some.  E.g., these `Key
(Character) Index' entries in the Emacs manual:

 * :d (Dired): Operating on Files.  (line 124)
 * :e (Dired): Operating on Files.  (line 136)
 * :s (Dired): Operating on Files.  (line 132)
 * :v (Dired): Operating on Files.  (line 128)

And _if you go to that index_ you can follow
those entries to their nodes, with no problem.

And we have command `info-apropos', which
checks all indexes of all manuals.  And with
that you can input a string that starts with
`:', and you can get to the locations targeted
by such entries with no problem.

The problem is only (?) with `i' (`Info-index').
With that you can type, say, `:d TAB', which
correctly shows you the completion `:d (Dired)'.

But choosing that completion takes you to the
wrong node - it finds 5 matches for `d (Dired)',
instead of one match for `:d (Dired)'.  (It does
find the desired match, but mixed in with other
noise - non-matches.)

2. If we can't fix this directly, then what
Michael A. says maybe points to a workaround
that Emacs could provide.

Many (most?) users won't have a clue about this 
difficulty of using `i' with index entries that
start with `:'.  And they won't understand
what's going on.

What we could do, is to tell a user, when s?he
tries to use `i' with a name that starts with
`:', that `i' only searches for index entries
with that name _minus_ the `:'.  We could then
suggest that s?he use `C-s :<whatever>' instead.

IOW, if this defect of our Info interface isn't
directly fixable then we might at least let
users know that (1) they can't reasonably use
`i :' and (2) they can at least use `C-s :' as
a workaround.