mm/gup.c: further document vma_permits_fault()

Describe the caller's responsibilities when passing
FAULT_FLAG_ALLOW_RETRY.

Link: http://lkml.kernel.org/r/1586915606.5647.5.camel@mtkswgap22


Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>