bug#31807: 27.0; `info-apropos' bad name or bad matching

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 12 Jun 2018 16:01:37 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> 
> `info-apropos', as its doc string says, apparently does NOT do "apropos"
> matching, i.e., regexp and keyword matching.  It apparently does literal
> string matching against index entries in all of the manuals.
> 
> Perhaps either the name should be changed (to not use the word
> "apropos") or the behavior should be changed, to allow apropos matching.

This command is the Emacs implementation of the "info --apropos"
feature present in the Texinfo's stand-alone Info reader, which also
looks for substring matches.  Apparently, "apropos" doesn't
necessarily mean "regexps and keywords".

> I threw this together quickly, as a POC.  It does the job, but

Thanks.  The original command was introduced 11 years ago, so I don't
think we can make radical changes in the user-visible behavior by
default after all this time.  This should be an opt-in optional
behavior, controlled either by a prefix argument or by a (new) user
variable.  Would you like to modify your suggestion along these lines?

Btw, does this really work as intended without changes to
Info-apropos-matches?  That function calls regexp-quote on its
argument.

> BTW, should the name be `Info-apropos' or `info-apropos'?

We've been through that, see the reason explained here:

  http://lists.gnu.org/archive/html/emacs-devel/2017-11/msg00482.html