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>

fs/proc/Kconfig

@@ -75,3 +75,9 @@ config PROC_PAGE_MONITOR
 config PROC_CHILDREN
 	bool "Include /proc/<pid>/task/<tid>/children file"
 	default n
+	help
+	  Provides a fast way to retrieve first level children pids of a task. See
+	  <file:Documentation/filesystems/proc.txt> for more information.
+
+	  Say Y if you are running any user-space software which takes benefit from
+	  this interface. For example, rkt is such a piece of software.