bug#20815: 25.0.50; doc of `null'

[Drew Adams]

The doc string should make clear that this is a Boolean
predicate.  Currently it says only "Return t if OBJECT is
nil.  It should also say "Return nil if OBJECT is non-nil.

The Elisp manual (node `List-related Predicates' (which should
be written `List-Related Predicates') is OK.  It says what the
return value is if OBJECT is not nil.



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'