Minor: Add documentation for cot

[alamb]

Which issue does this PR close?

Part of #12740

Rationale for this change

I noticed that the cot function does not have documentation by running the script in #12872

(venv) andrewlamb@Andrews-MacBook-Pro-2:~/Software/datafusion$ ./dev/update_function_docs.sh
...
Running CLI and inserting scalar function docs table
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.25s
     Running `target/debug/print_functions_docs scalar`
INFO: The following functions do not have documentation:
  - map_extract
  - map
  - cot
  - map_keys
  - map_values

Note that @ jonathanc-n is working on the map functions in #13047

In fact it appears to be completely undocumented in the old docs too: https://datafusion.apache.org/user-guide/sql/scalar_functions.html so this is a win for the automated script

What changes are included in this PR?

Add docs to cot

Are these changes tested?

Yes by CI

Are there any user-facing changes?

More docs!

[jonahgao]

LGTM👍

[alamb]

Thank you @jonahgao for the review

[alamb]

I reran the doc generator locally to make sure there are no logical conflicts and things looked good