bug#13926: 24.3.50; doc string of `visual-line-mode'

[Drew Adams]

The doc string is misleading wrt the argument when called from Lisp.
 
 With a prefix argument ARG, enable Visual Line mode if ARG is
 positive, and disable it otherwise.  If called from Lisp, enable
 the mode if ARG is omitted or nil.
 
This makes no connection between the interactive prefix arg and the arg
when called from Lisp.  In particular, it can also give the incorrect
impression that the mode is enabled ONLY if ARG is omitted or nil.
There is nothing that suggests the behavior of a non-positive or
positive ARG when called from Lisp.
 
By way of contrast, the doc string of `define-minor-mode' correctly says
what the argument values do when called from Lisp:
 
 When called from Lisp, the mode command toggles the mode if the
 argument is `toggle', disables the mode if the argument is a
 non-positive integer, and enables the mode otherwise (including
 if the argument is omitted or nil or a positive integer).
 
This kind of careful description should probably be added automatically
to functions defined by `define-minor-mode', to prevent misleading doc
wrt the argument.

In GNU Emacs 24.3.50.1 (i386-mingw-nt5.1.2600)
 of 2013-03-04 on ODIEONE
Bzr revision: 111935 yamaoka@jpl.org-20130304102733-4qy111z41qwoh2as
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.7) --no-opt --enable-checking --cflags
 -IC:/Devel/emacs/build/include --ldflags -LC:/Devel/emacs/build/lib'