Documentation/kmemleak.txt: updates

Update Documentatin/kmemleak.txt to reflect the following changes:

Commit b69ec42b ("Kconfig: clean up the long arch list for the
DEBUG_KMEMLEAK config option") made it so that we can't check supported
architectures by read Kconfig.debug.

Commit 85d3a316 ("kmemleak: use rbtree instead of prio tree")
converted kmemleak to use rbtree instead of prio tree.

Signed-off-by: Wang YanQing <udknight@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>