bug#12846: 24.2.50; file-cach-* commands that use `(interactive "X...")'

[Drew Adams]

The doc, and perhaps the prompt as well, should tell users that their
input must be a Lisp sexp that is read and evaluated to produce a
DIRECTORY-LIST.  It makes no sense to tell them only that they are to
input a list of directory names.
 
There are no doubt other commands that use `(interactive "X...")' which
also need such clarification.  I just happened to notice that the
file-cache commands provide no help for users in this regard.

In GNU Emacs 24.2.50.1 (i386-mingw-nt5.1.2600)
 of 2012-10-22 on DANI-PC
Bzr revision: 110618 monnier@iro.umontreal.ca-20121022132928-232zm0fecassmhfb
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.7) --no-opt --enable-checking --cflags
 -I../../libs/libxpm-3.5.8/include -I../../libs/libxpm-3.5.8/src
 -I../../libs/libpng-1.4.10 -I../../libs/zlib-1.2.6
 -I../../libs/giflib-4.1.4-1/include -I../../libs/jpeg-6b-4/include
 -I../../libs/tiff-3.8.2-1/include
 -I../../libs/libxml2-2.7.8-w32-bin/include/libxml2
 -I../../libs/gnutls-3.0.16/include
 -I../../libs/libiconv-1.14-2-mingw32-dev/include'