documentation: erasure code profile update

The plugins are made a sub-section of the erasure code profile
section. The k and m parameters are removed from erasure code profile
documentation. They were added for simplicity in the first version
because there was only one plugin. But they should really be in the
plugin documentation.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
documentation: isa plugin is sub section of erasure code profile

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
documentation: jerasure plugin is sub section of erasure code profile

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2482 from dachary/wip-erasure-code-documentation

erasure code documentation

Reviewed-by: Sage Weil <sage@redhat.com>