explain what an inline xattr is and how it relates to omap

The logic of the configuration flags related to xattr is clarified to define what an inline xattr is and when storing in the object map is preferred.

http://tracker.ceph.com/issues/4617 refs #4617

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #184 from dachary/wip-4617

explain what an inline xattr is and how it relates to omap