bug#18318: 24.4.50; doc of `file-readable-p'

[Drew Adams]

The doc - both Elisp manual and doc string, should explicitly point out
that this returns non-nil for a directory, IOW, a non-nil value does not
mean that the file can be, say, inserted into a buffer.

In addition, it would be better to change the parameter name to
FILE-OR-DIR (or even FILE-OR-DIR-NAME).  It is all to easy to not
realize, when reading this, that this does not test for a non-directory
file that is readable.

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''