kcore: add Kconfig help text

Under Pseudo filesystems, /proc/kcore support has no help.

Fixes a portion of kernel bugzilla #52671:
  https://bugzilla.kernel.org/show_bug.cgi?id=52671

Thanks for David Howells for the help text.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by:  <lailavrazda1979@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>