This patch updates docs/qcow2-cache.txt explaining how to use the new
l2-cache-entry-size parameter.

Here's a more detailed technical description of this feature:

   https://lists.gnu.org/archive/html/qemu-block/2017-09/msg00635.html

And here are some performance numbers:

   https://lists.gnu.org/archive/html/qemu-block/2017-12/msg00507.htmlSigned-off-by: NAlberto Garcia <berto@igalia.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>