Use XML revision tag for extensions to generate specification revisions

PR https://github.com/KhronosGroup/OpenCL-Docs/pull/1161 enforces that every extension defined by the XML must have a revision associated with it.

This revision will be used to generate an extension version in the headers, but it could also be used to generate the revision set in the human readable OpenCL specification. Currently, if an implementer wants to update the version of an extension they need to remember to update **both** the specification document and the XML entry. However, it would be less error prone to have the XML as the single source of truth that is used to generate the rendered specification document.

This may be blocked on https://github.com/KhronosGroup/OpenCL-Docs/issues/1168 so that all the revision XML entries are accurate, as we don't want to use the placeholder values in the rendered spec. Although we could check and only use the XML revision in the rendered specification if it isn't a placeholder value.

Start by tracing how XML extension entries are consumed when the human-readable specification is rendered. Check issue 1168 before using revision values, and compare the generated revision set with the XML source. Done means the rendered specification derives extension revisions from XML without using placeholder values.

Written by the indexing model from the issue text.