Migrate documentation for `regr*` aggregate functions to code

[alamb]

Which issue does this PR close?

Part of https://github.com/apache/datafusion/issues/12827

Rationale for this change

https://github.com/apache/datafusion/pull/12861 from @jonathanc-n moves some of the aggregate function documentation to the code, but the regr functions are handled specially

What changes are included in this PR?

Move regr_* aggregate functions to code

Are these changes tested?

Yes y CI

Are there any user-facing changes?

[Omega359]

Beyond the one minor change this LGTM

[alamb]

Thank you for the review @korowa

[alamb]

🚀 let's do this. Thanks again @goldmedal and @korowa