bug#11563: 24.0.97; doc string of command `dired-insert-subdir'

[Drew Adams]

The doc string says (correctly):

"With a prefix arg, you may edit the `ls' switches used for this listing.
 You can add `R' to the switches to expand the whole tree starting at
 this subdirectory."

However, it seems like it should also mention that an error will be
raised if the switches you specify are incompatible with the switches in
the Dired buffer.

But that begs the question of what those incompatibilities are.  We
should tell users in the doc string just what they cannot do, instead
of making them discover this by trial and error.

According to `dired-insert-subdir-validate' the incompatibilities are
for `F' and `b': a Dired buffer cannot have listings with and without
`F' or with and without `b'.  This should be said in the doc string of
`dired-insert-subdir'.

In GNU Emacs 24.0.97.1 (i386-mingw-nt5.1.2600)
 of 2012-05-16 on MARVIN
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.6) --no-opt --enable-checking --cflags
 -ID:/devel/emacs/libs/libXpm-3.5.8/include
 -ID:/devel/emacs/libs/libXpm-3.5.8/src
 -ID:/devel/emacs/libs/libpng-dev_1.4.3-1/include
 -ID:/devel/emacs/libs/zlib-dev_1.2.5-2/include
 -ID:/devel/emacs/libs/giflib-4.1.4-1/include
 -ID:/devel/emacs/libs/jpeg-6b-4/include
 -ID:/devel/emacs/libs/tiff-3.8.2-1/include
 -ID:/devel/emacs/libs/gnutls-3.0.9/include'