[Clang][TableGen] Sort undocumented builtins after documented ones in generated docs (#183938)
The builtin documentation emitter previously sorted all categories
purely alphabetically, which placed the "Undocumented" section before
categories like "WMMA" in the generated RST. This made the output
confusing since stub entries appeared before real documentation.
Push the "Undocumented" category to the end of the output so that all
documented categories appear first, regardless of their names.