bug#28337: 25.2; make clear(er) that `visual-line-mode' is not global

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

It doesn't seem clear enough, from either the doc string or (emacs)
Visual Line Mode, that the mode is not global.  There is some mention of
"the buffer", but that could easily be read as referring to the current
buffer in the context of a global minor mode.

It would help if the doc stated explicitly that the minor mode is local
to the current buffer.  This might, in fact, be the case for other
(all?) non-global minor modes.

Originally global minor modes were relatively rare.  I think this may no
longer be the case (in number they are likely still less common than
non-global ones, but perhaps not in frequency of use - dunno).  The doc
for a global or globalized minor mode makes clear that the effect is
global. I'm not sure that the doc for a non-global one makes clear that
the effect is only for the current buffer.

Here is one possible case of confusion by a user.  (Not sure there was
this confusion; it could be just not knowing about hooks.)

https://emacs.stackexchange.com/q/35263/105


In GNU Emacs 25.2.1 (x86_64-w64-mingw32)
 of 2017-04-24 built on LAPHROAIG
Windowing system distributor 'Microsoft Corp.', version 6.1.7601
Configured using:
 'configure --without-dbus --without-compress-install 'CFLAGS=-O2
 -static -g3''