mm: add documentation describing what tsk->active_mm means vs tsk->mm

I'm sure everyone knows this, but I didn't, so I googled it, and found a
nice explanation from Linus.  Might be worth sticking in Documentation.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>