erasure code : plugin, interface and glossary documentation updates

* replace the erasure code plugin abstract interface with a doxygen link
  that will be populated when the header shows in master
* update the plugin documentation to reflect the current draft implementation
* fix broken link to PGBackend-h
* add a glossary to define chunk, stripe, shard and strip with a drawing

http://tracker.ceph.com/issues/4929 refs #4929

Signed-off-by: Loic Dachary <loic@dachary.org>