blk-crypto: update inline encryption documentation

Rework most of inline-encryption.rst to be easier to follow, to correct
some information, to add some important details and remove some
unimportant details, and to take into account the renaming from
blk_keyslot_manager to blk_crypto_profile.

Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20211018180453.40441-5-ebiggers@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>