Documentation/features: Add kmemleak

Replace the manually curated list for supported archs in
Documentation/dev-tools/kmemleak.rst by a Documentation/features entry.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Link: https://lore.kernel.org/r/20200627091510.28210-1-tklauser@distanz.ch


Signed-off-by: Jonathan Corbet <corbet@lwn.net>