Document and clean up CodegenParams (#51140)
- Add documentation to CodegenParams fields per request in #51123
- Fix compare_cgparams which hadn't been updated for recent additions
- Remove unused and untested generic_context
The latter was a codegen option that I added, but eventually ended up
not using anywhere, so remove it for the time being. That said, I may
end up in a situation where I need it again in the very near future, so
I may end up eating my words here, but if I need to put it back, I'll
include a test at least ;).
