increase usage of asciidoctor dictionaries in extensions spec

We moved the OpenCL API spec to use asciidoctor dictionaries for API names, type names, and enum names several years ago, but we haven't made similar upgrades to the extension spec. Switching to the asciidoctor dictionaries will provide a similar look-and-feel across all extensions and will also provide a useful check that all of the symbols added by an extension are properly spelled and that we haven't omitted any symbols from the XML file.

I'll probably do these conversions one at a time to provide for easier review and so we can potentially parallelize the work. We can use this issue to track progress and what is remaining.