We're missing a description for the 'nr_vecs' parameter. While in there,
clarify that freeing a bio allocated through this function must be done
from process context.

Fixes: 1cbbd31c4ada ("bio: add allocation cache abstraction")
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NJens Axboe <axboe@kernel.dk>