man: document new register/update API
man: document new register/update API

Document
- IORING_REGISTER_FILES2
- IORING_REGISTER_FILES_UPDATE2,
- IORING_REGISTER_BUFFERS2
- IORING_REGISTER_BUFFERS_UPDATE,

And add a couple of words on registered resources (buffers, files)
tagging.

Signed-off-by: Pavel Begunkov <8608d41fe50396a9dbd36e830be65ba791e26c05@gmail.com>
Link: https://lore.kernel.org/r/f456c80bb8795eb7b8c3db8279206d94ce148587.1630436406.git.8608d41fe50396a9dbd36e830be65ba791e26c05@gmail.com
[axboe: minor edits]
Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing