Add revision to XML extensions (KhronosGroup#1161)

This extension adds the revision field to the
XML tag for extensions. This allows a version
macro to be generated with:

* KhronosGroup/OpenCL-Headers#251
* KhronosGroup/OpenCL-Headers#248

KHR extensions are given a revision based on the semantic
version of the spec. However other extensions don't use
semantic versioning, and so are given a placeholder `0.0.0`
value until they can be updated by the owner.

The XML schema is also updated to make the revision field
mandatory in the XML entry for extensions and the
existence of the macro this enables is advertised to users
in the spec.

api/opencl_architecture.asciidoc

@@ -2263,6 +2263,25 @@ typedef cl_uint cl_version;
     ((patch) & CL_VERSION_PATCH_MASK))
 ----
 
+[NOTE]
+--
+The available version of an extension is exposed to the user via a macro
+defined by the OpenCL Headers. This macro takes the format of the uppercase
+extension name followed by the `_EXTENSION_VERSION` suffix. For example,
+`CL_KHR_SEMAPHORE_EXTENSION_VERSION` is the macro defining the version of the
+{cl_khr_semaphore_EXT} extension.
+
+The value of this macro is set to the {cl_version_TYPE} of the extension using
+the semantic version of the extension. If no semantic version is defined for
+the extension, then the value of the macro is set to `0` to represent semantic
+version `0.0.0`.
+
+Applications can use these version macros along with the convience macros
+defined in this section to guard their code against breaking changes to the API
+of extensions, in particular provisional KHR extensions which have yet to
+finalize an API.
+--
+
 [[version-name-pairing]]
 ==== Version-Name Pairing