"Style guide" for the specs

[oddhack]

Spinning off some side discussion on #950, it would be helpful for future contributors to document some of the repo-specific markup, especially the varying forms of {clAPIname} macros used to generate xrefs and links. Some of the Vulkan style guide is applicable, like the descriptions of open block markup from whence refpages are extracted, as well. While this can mostly be inferred by pattern-matching, documentation is better. Need not be nearly as extensive as the Vulkan guide. I could contribute to this if that would be helpful, but it's mostly a spec editor thing.