documentation: isa plugin is sub section of erasure code profile

Signed-off-by: Loic Dachary <loic-201408@dachary.org>

diff --git a/doc/rados/operations/erasure-code-isa.rst b/doc/rados/operations/erasure-code-isa.rst
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+=======================
+ISA erasure code plugin
+=======================
+
+The *isa* plugin is encapsulates the `ISA
+<https://01.org/intel%C2%AE-storage-acceleration-library-open-source-version/>`_
+library. It only runs on Intel processors.
+
+Create an isa profile
+=====================
+
+To create a new *jerasure* erasure code profile::
+
+        ceph osd erasure-code-profile set {name} \
+             plugin=isa \
+             k={data-chunks} \
+             m={coding-chunks} \
+             technique={reed_sol_van|cauchy} \
+             [ruleset-root={root}] \
+             [ruleset-failure-domain={bucket-type}] \
+             [directory={directory}] \
+             [--force]
+
+Where:
+
+``k={data chunks}``
+
+:Description: Each object is split in **data-chunks** parts,
+              each stored on a different OSD.
+
+:Type: Integer
+:Required: Yes.
+:Example: 4
+
+``m={coding-chunks}``
+
+:Description: Compute **coding chunks** for each object and store them
+              on different OSDs. The number of coding chunks is also
+              the number of OSDs that can be down without losing data.
+
+:Type: Integer
+:Required: Yes.
+:Example: 2
+
+``technique={reed_sol_van|cauchy}``
+
+:Description: The ISA plugin comes in two `Reed Solomon
+              <https://en.wikipedia.org/wiki/Reed%E2%80%93Solomon_error_correction>`_
+              forms. If *reed_sol_van* is set, it is `Vandermonde
+              <https://en.wikipedia.org/wiki/Vandermonde_matrix>`_, if
+              *cauchy* is set, it is `Cauchy
+              <https://en.wikipedia.org/wiki/Cauchy_matrix>`_.
+
+:Type: String
+:Required: No.
+:Default: reed_sol_van
+
+``ruleset-root={root}``
+
+:Description: The name of the crush bucket used for the first step of
+              the ruleset. For intance **step take default**.
+
+:Type: String
+:Required: No.
+:Default: default
+
+``ruleset-failure-domain={bucket-type}``
+
+:Description: Ensure that no two chunks are in a bucket with the same
+              failure domain. For instance, if the failure domain is
+              **host** no two chunks will be stored on the same
+              host. It is used to create a ruleset step such as **step
+              chooseleaf host**.
+
+:Type: String
+:Required: No.
+:Default: host
+
+``directory={directory}``
+
+:Description: Set the **directory** name from which the erasure code
+              plugin is loaded.
+
+:Type: String
+:Required: No.
+:Default: /usr/lib/ceph/erasure-code
+
+``--force``
+
+:Description: Override an existing profile by the same name.
+
+:Type: String
+:Required: No.
+