[ doc ] document %cg pragma

[dunhamsteve]

Description

Add documentation for the %cg pragma. I missed this one when documenting the pragmas because it is hiding in the lexer.

[dunhamsteve]

I meant to comment on that review, but accidentally hit resolve (I prefer to let the original reviewer resolve). The directives for all supplied codegen are stored in the ttc, and directives for the current codegen are used when compiling. How they are aggregated depends on the directive, but extraRuntime in chez is collected and deduplicated. I've updated the docs to include this information.

[andrevidela]

That looks good to me! Any reason we should delay this?