aoe: update documentation with new URL and VM settings reference

The old area has a new URL.  Also, now that the driver can perform better,
it is worth mentioning the VM settings that help aoe to sink dirty pages
out early, avoiding unecessary memory pressure when much I/O is going on.

Signed-off-by: Ed Cashin <ecashin@coraid.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>