documentation: erasure-code plugin isa does not require k/m

http://tracker.ceph.com/issues/9579 Refs: #9579

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2571 from dachary/wip-9579-isa-documentation

documentation: erasure-code plugin isa does not require k/m

Reviewed-by: Andreas Peters <andreas.joachim.peters@cern.ch>