block: improve the bio_add_page() and bio_add_pc_page() descriptions

The descriptions of bio_add_page() and bio_add_pc_page() are slightly
inconsistent; improve them.

Signed-off-by: Andreas Gruenbacher <agruen@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>