Add revision to XML extensions

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 +2263,24 @@ typedef cl_uint cl_version;
  ((patch) & CL_VERSION_PATCH_MASK))
 ----
 
+[NOTE]
+--
+The OpenCL headers make the version of an extension defined in the headers
+available using a macro. This macro takes the format of the uppercase extension
+name followed by the `EXTENSION_VERSION` suffix. For example,
+`CL_KHR_CREATE_COMMAND_QUEUE_EXTENSION_VERSION` is the macro defining the
+version of the {cl_khr_command_queue_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` (semantic version
+`0.0.0`).
+
+Applications can use this macro to guard their code against API breaking
+changes to the API of extensions, in particular provisional KHR extensions
+which have yet to finalize the API.
+--
+
 [[version-name-pairing]]
 ==== Version-Name Pairing