bug#32889: 26; `savehist-mode`: Make clear in the doc string that you can use it to save any variables

[Drew Adams]

Option `savehist-additional-variables' should be mentioned in the doc
string.

By default `savehist-mode' saves only ordinary minibuffer histories, and
that's not the case for `command-history', for example.  Users have been
known to ask how to save the `M-x' history across sessions.

Beyond, `command-history', you can tell `savehist-mode' to persist any
variables.  That possibility should be mentioned in the doc, as its
scope is far beyond what `savehist-mode' does by default.

In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.16299
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''