bug#52901: 26.3; (emacs) `Completion Styles': provide more guidance

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

In this node each predefined completion style is described.  But these
two bits of guidance are missing - please consider adding them:

1. Mention `completion-styles-alist', as does the doc string of
   `completion-styles'.  (We do mention `completion-category-overrides',
   at least.)

2. Say something about combinations of the styles.  The completion
   behavior depends on the combination that's the value of option
   `completion-styles'.

   It's not enough to describe each predefined style individually, and
   say that they are tried in order.  Please consider providing some
   comparison of the predefined styles: which of them in some sense
   include or subsume (or partially so) which others.  Give users an
   idea of which they don't need to include if they include this or that
   other.

In general, a little more guidance for users, please.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.19042
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''