EC: document the LRC per layer plugin configuration

LRC now uses Jerasure as the default EC backend. But it is actually
possible to switch to other backend like Isa using the low level
configuration. This commits Adds documents on how to specify the EC
backend in each LRC layer:

Signed-off-by: Yuan Zhou <yuan.zhou@intel.com>
Merge pull request #2781 from zhouyuan/lrc_layer_plugin_doc

EC: document the LRC per layer plugin configuration

Reviewed-by: Loic Dachary <loic-201408@dachary.org>