Documentation: Add `related_udfs` and `alternative_syntax` to doc gen macros

[comphead]

          Thanks @Omega359 I will be adding the udfs and alternate syntax in follow up PR. I'll find the UDFs supporting both `related_udfs` and `alternative_syntax` and make these changes simultaneously along with some other things. Preferring the follow up PR as this one is already too large to review

Originally posted by @comphead in #12822 (comment)