bug#14289: 24.3.50; doc string of `find-program'

[Drew Adams]

It's not just about `grep-find-command'.  And its value does not just
take "effect when `grep-compute-defaults' is called."
 
Th variable is used also in other Emacs commands: including
`find-dired', `find-name-dired', and `find-grep-dired'.
 
 
In GNU Emacs 24.3.50.1 (i386-mingw-nt5.1.2600)
 of 2013-04-26 on ODIEONE
Bzr revision: 112399 eggert@cs.ucla.edu-20130426193109-3chlhmjv621m7zdz
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.7) --no-opt --enable-checking --cflags
 -IC:/Devel/emacs/build/include --ldflags -LC:/Devel/emacs/build/lib'