bug#19209: 25.0.50; doc string of `mwheel-scroll' is incorrect

[Drew Adams]

emacs -Q

C-h f mwheel-scroll provides contradictory information - on MS Windows,
at least:

  It is bound to S-wheel-down, wheel-down, S-wheel-up, wheel-up.

  (mwheel-scroll EVENT)

  Scroll up or down according to the EVENT.
  This should only be bound to mouse buttons 4 and 5.

According to this, Emacs -Q is doing something it "should" not do:
binding the command to `S-wheel-down', `wheel-down', `S-wheel-up', and
`wheel-up'.

Please remove the platform-specific sentence that says that this should
only be bound to `mouse-4' and `mouse-5'.

(That text was anyway worded poorly - it should have said "This should
be bound only to `mouse-4' and `mouse-5'.")

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'