bug#21695: 25.0.50; Change most occurrences of `setq' in Emacs manual to `customize-set-variable'

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

The same is apparently true for the manual "An Introduction
to Programming in Emacs Lisp" (which is a fine intro to learning
Elisp).

What's more, it even states:

 "The 'custom-set-variables' function works somewhat differently than a
 'setq'.  While I have never learned the differences, I modify the
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 'custom-set-variables' expressions in my '.emacs' file by hand: I make
 the changes in what appears to me to be a reasonable manner and have not
 had any problems.  Others prefer to use the Customization command and
 let Emacs do the work for them."

It might have been OK to say that when it was first written,
but by now the manual should understand the differences, and
even point them out.  Especially in that section, which is
about `defcustom'.  The node says:

"Although you can use 'defvar' or 'setq' for variables that
users set, the 'defcustom' macro is designed for the job."

Part of its design for that job is handling `:type', `:set'
`:initialize', et.c, precisely what makes options different
from ordinary variables and thus `custom-set-variables'
different from `setq'.