fs, proc: add help for CONFIG_PROC_CHILDREN

The purpose of the option was documented in
Documentation/filesystems/proc.txt but the help text was missing.

Add small help text that also points to the documentation.

Signed-off-by: Iago López Galeiras <iago@endocode.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>