bug#18211: 24.4.50; doc string of `posnp'

[Drew Adams]

Doc string:

 Return non-nil if OBJ appears to be a valid `posn' object.

What is a `posn' object?  What is a valid one?

(elisp) `Accessing Mouse' gives an understandable description.  If it is
correct then use something similar for the doc string:

     returns non-`nil' if OBJECT is a mouse position list,
                                    ^^^^^^^^^^^^^^^^^^^^^
     in either of the formats documented in *note Click
     Events::); and `nil' otherwise.

(FWIW, that manual text should not say "either", since there are more
than two such formats.  It should say "one".)



In GNU Emacs 24.4.50.1 (i686-pc-mingw32)
 of 2014-06-28 on ODIEONE
Bzr revision: 117431 rgm@gnu.org-20140628015517-eku6hj8mpgcvfnso
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/snapshot/trunk
 --enable-checking=yes,glyphs 'CFLAGS=-O0 -g3'
 LDFLAGS=-Lc:/Devel/emacs/lib 'CPPFLAGS=-DGC_MCHECK=1
 -Ic:/Devel/emacs/include''