[Async UDF] Add high level context / examples for user defined async functions

[alamb]

• Part of [EPIC] More Async User Defined Function work #16520

It would be nice to add some high level context to this example -- like an introduction saying that most functions are sync, but for some functions can be run as async ...

I can help with this potentially.

It would also be awesome to put this example / code in the docs https://datafusion.apache.org/library-user-guide/adding-udfs.html so it was easier to find

Originally posted by @alamb in #14837 (comment)

[alamb]

Actually I see this is now done as part of #14837