Add CommonMark variant

Author: msabramo

source/specifications.rst

@@ -94,10 +94,16 @@ UTF-8.
 
 Other parameters might be specific to the chosen subtype. For example, for the
 ``markdown`` subtype, there is a ``variant`` parameter that allows specifying
-the variant of Markdown in use, such as ``Original`` for `Gruber's original
-Markdown syntax <https://tools.ietf.org/html/rfc7763#section-6.1.4>`_ or
-``GFM`` for `GitHub Flavored Markdown (GFM)
-<https://tools.ietf.org/html/rfc7764#section-3.2>`_.
+the variant of Markdown in use, such as:
+
+- ``CommonMark`` for `CommonMark`
+  <https://tools.ietf.org/html/rfc7764#section-3.5>`_
+
+- ``GFM`` for `GitHub Flavored Markdown (GFM)
+  <https://tools.ietf.org/html/rfc7764#section-3.2>`_
+
+- ``Original`` for `Gruber's original Markdown syntax
+  <https://tools.ietf.org/html/rfc7763#section-6.1.4>`_
 
 Example::
 
@@ -109,12 +115,16 @@ Example::
 
 Example::
 
-    Description-Content-Type: text/markdown; charset=UTF-8; variant=Original
+    Description-Content-Type: text/markdown; charset=UTF-8; variant=CommonMark
 
 Example::
 
     Description-Content-Type: text/markdown; charset=UTF-8; variant=GFM
 
+Example::
+
+    Description-Content-Type: text/markdown; charset=UTF-8; variant=Original
+
 If a ``Description-Content-Type`` is not specified, then the assumed content type
 is ``text/x-rst; charset=UTF-8``.