bug#13339: 24.3.50; doc of `next-frame'

[Drew Adams]

The doc (both doc string and Elisp manual) is unclear for the default
case where argument MINIFRAME/MINIBUF is nil.  (BTW, the same parameter
name should be used here for the doc string and the manual.)
 
What is unclear is which frames are _included_ in this case.  The only
thing we are told is that minibuffer-only frames are _excluded_.  Does
that mean that all other frames are included?  Or only visible frames?
Or only visible or iconified frames?
 
Other values of this argument make clear which frames are included, but
the doc for a nil value of the argument is incomplete.
 
In GNU Emacs 24.3.50.1 (i386-mingw-nt5.1.2600)
 of 2012-12-31 on ODIEONE
Bzr revision: 111388 rudalics@gmx.at-20121231113513-subz2dazg6yjukzh
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'