adding manual cross-ref links to *Help*

[Drew Adams]

I've added cross-reference links to the manuals from *Help* buffers.  This text
is added:

  For more information check the manuals.

`manuals' here is a link to an Info virtual menu of links to the indexed
occurrences (of the help target) in the manuals.

For example, if you do 'C-h f forward-char' and then click the `manuals' link
you get an Info buffer that is a menu of the index entries for 'forward-char' in
all of the manuals searched.  In the case of function 'forward-char' there are
two links:

  * forward-char [elisp]:   (elisp)Character Motion.
  * forward-char [emacs]:   (emacs)Moving Point.

I can submit a patch for this if people are interested.  To try it out first,
just load help-fns+.el, which you can find here:
http://www.emacswiki.org/emacs/download/help-fns%2b.el

User option `help-cross-reference-manuals' controls which manuals to search.  By
default, only the Emacs and Elisp manuals are searched.  If you customize it to
search all manuals present, then the `C-h f forward-char' example also includes
this entry (on my system):

  * forward-char [mh-e]:    (mh-e)Processing Mail Tour.

The same user option controls whether to also search the manuals when composing
*Help*, thus to avoid creating the manuals link if there are in fact no index
entries for the given help target.  By default (to save time), the link is
always created - there is no manuals search until you click the link.


[Note (esp. to Juri):
Info already has an apropos search and an index menu.  But the former includes
hits that are not literal (exact) matches, and the latter is only for a single
manual AFAICT.  So I added functions `Info-indexed-find-file' and
`Info-indexed-find-node' and added them to `Info-virtual-files'.  It might be
better (dunno) to instead extend `Info-virtual-index' to work across multiple
manuals etc.]