bug#2035: 23.0.60; doc string of dired-read-shell-command

[Drew Adams]

Doc string:
 
"Read a dired shell command prompting with PROMPT (using read-string).
ARG is the prefix arg and may be used to indicate in the prompt which
  files are affected.
This is an extra function so that you can redefine it."
 
1. The part about the "prefix arg", at least, doesn't make sense. This
is not a command, and there is no reference in the code to
`current-prefix-arg'. The sentence is not clear to me, even ignoring
"prefix" - *how* does ARG indicate, in the prompt, which files are
affected?
 
2. Argument FILES is not described (should be uppercase).
 
3. It's not clear (to me) what the last sentence is supposed to mean.

In GNU Emacs 23.0.60.1 (i386-mingw-nt5.1.2600) of 2009-01-04 on
 LENNART-69DE564 Windowing system distributor `Microsoft Corp.',
 version 5.1.2600 configured using `configure --with-gcc (3.4)
 --no-opt --cflags -Ic:/g/include -fno-crossjumping'