No reference documentation for `cwltool:CUDARequirement`

There is no current definition in https://www.commonwl.org/v1.2/CommandLineTool.html for `cwltool:CUDARequirement`.
Considering it is the *"core"* CWL specification, this is understandable.

However, it is mentioned in http://www.commonwl.org/user_guide/introduction/basic-concepts.html#processes-and-requirements
and other tools have also added support for it:
- https://doc.arvados.org/v2.4/user/cwl/cwl-extensions.html#CUDARequirement
- https://github.com/DataBiosphere/toil/pull/4223
- https://github.com/crim-ca/weaver/pull/494

It would be very helpful to have an official location where this is documented.
It could be placed in an "extensions" section to highlight that it is not *core* CWL.

Start with the user guide's Processes and Requirements section and review the linked Arvados CUDARequirement documentation and related tool support. Add an official, clearly marked extension reference for cwltool:CUDARequirement; done when users can find its definition without mistaking it for core CWL.

Written by the indexing model from the issue text.