doc : erasure code developer notes updates

* unify conventions to match those used by jerasure ( data chunk = K,
  coding chunk = M, use coding instead of parity, use erasures instead
  of erased )

* make lines 80 characters long

* modify the descriptions to take into account that the chunk rank
  will encoded in the pool name and not on a per object basis

* remove the doxygen link to ErasureCodeInterface because it fails
  doc: asphyxiate does not support class
  http://tracker.ceph.com/issues/6115

* only systematic codes are considered at this point ( all jerasure
  techniques are systematic). Although the API could be extended to
  include non systematic codes, it is probably a case of over
  engineering at this point.

* add link to
  http://tracker.ceph.com/issues/6113
  add ceph osd pool create [name] [key=value]

* update the plugin system description to match the proposed
  implementation http://tracker.ceph.com/issues/5877

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

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #539 from dachary/master

doc : erasure code developer notes updates