Improve doc comments inside macros

[Rafferty97]

Summary

Some items defined within macros use macro tokens like $UDF within their doc comments, which render as that literal string in cargo documentation, which is confusing and looks a little broken. Here's an example:

https://docs.rs/datafusion/latest/datafusion/functions/math/fn.cot.html

Proposal

Use a combination of #[doc], concat! and stringify! to properly interpolate relevant type names into the generated doc comments.