CL_VERSION_2_1 and CL_VERSION_2_2 missing in documentation

It is documented, that one can test the api version up to OpenCL 2.0 by testing macros like `CL_VERSION_2_0`. However the macros `CL_VERSION_2_1` and `CL_VERSION_2_2` are missing in the documentation, although they are defined in the header. Are these makros guarantied to be defined (in OpenCL 2.2) and just missing from the documentation or could there be conforming implementations, that don't define those?

Note: even the makros up to `CL_VERSION_2_0` are only documented in the [Appendix E: Changes to OpenCL](https://github.com/KhronosGroup/OpenCL-Docs/blob/master/api/appendix_e.asciidoc) and not in the main text.

Note 2: The string `CL_VERSION_2_2` does appear in [xml/cl.xml on line 4303](https://github.com/KhronosGroup/OpenCL-Docs/blob/master/xml/cl.xml#L4303) but I don't really get if that does mean, that a macro of that name is defined...

Start with Appendix E: Changes to OpenCL and the CL_VERSION_2_2 entry in xml/cl.xml, then compare how CL_VERSION_2_0 is documented. Confirm the intended guarantee for CL_VERSION_2_1 and CL_VERSION_2_2 and update the relevant documentation, including the main text if appropriate. Done means the macro definitions and conformance expectations are stated clearly.

Written by the indexing model from the issue text.