doc string of `diff' does not mention prefix arg

[Drew Adams]

The doc string of command `diff' should mention that if you supply a
prefix arg you are prompted for the diff switches.  Otherwise, there
is no way to know this.  If you somehow figure out that a prefix arg
calls function `diff-switches', you still can't know what it does,
because there is no doc string for `diff-switches' - it should have a
doc string.


In GNU Emacs 22.1.1 (i386-mingw-nt5.1.2600)
 of 2007-06-02 on RELEASE
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4) --cflags -Ic:/gnuwin32/include'