doc/rbd: Document new RBD feature flags and librbd/krbd version support

doc/rbd: Document new RBD feature flags and librbd/krbd version support

Sources:

layering:

* https://github.com/ceph/ceph/commit/f87c441f6891e5e72d5e712d9a19e45a1b9a97eb
* krbd: https://github.com/torvalds/linux/commit/770eba6e295fd36e43881176ee0644b9cc2803f1

striping:

* https://github.com/ceph/ceph/commit/2e403ef2c98bb64220060c06b435fb1ffe08294e
* krbd: https://github.com/torvalds/linux/commit/770eba6e295fd36e43881176ee0644b9cc2803f1

exclusive-lock:

* https://github.com/ceph/ceph/commit/fa96bb4db47c500947efcab51892f45a04b03d30
* krbd: https://github.com/torvalds/linux/commit/ed95b21a4b0a71ef89306cdeb427d53cc9cb343f

object-map:

* https://github.com/ceph/ceph/commit/7e751ce9125cbeb04b9551afff4b91c6255c03f9

fast-diff:

* https://github.com/ceph/ceph/commit/16ad44cebcc5d2464be38b95f16e0d63d2b29ed1

deep-flatten:

* https://github.com/ceph/ceph/commit/b556d3173ae4483e681bb21658a7d484a2627386

journaling:

* https://github.com/ceph/ceph/commit/750771c6ff07f6fcf0235172ec68ff4d907c621d
* not a default: https://github.com/ceph/ceph/blob/1a14963/src/test/cli/rbd/help.t#L229

data-pool:

* https://github.com/ceph/ceph/commit/c5f22900474e0eed88910341de57e17e1cc154f0
* description: https://tracker.ceph.com/issues/17409
* krbd: https://github.com/torvalds/linux/commit/7e97332ea9caad3b7c6d86bc3b982e17eda2f736

operations:

* https://github.com/ceph/ceph/commit/209f6fc5a44e2d068ce0dd8b3e9a25ee700ad0e0
* description: https://github.com/torvalds/linux/commit/e573427a440fd67d3f522357d7ac901d59281948
* krbd: https://github.com/torvalds/linux/commit/e573427a440fd67d3f522357d7ac901d59281948

migrating:

* https://github.com/ceph/ceph/commit/2f7c4ae3fc7ec7a2bfe99e116bf5e4f71f24c6a1

Signed-off-by: Valentin Lorentz <progval+git@progval.net>