regexp_extract docs should highlight its peculiarities

[ggevay]

The common theme across all table functions is that they can compute multiple output values for each input row. But regexp_extract seems to be a really special table function in how it returns its multiple output values. Most table functions put their multiple output values in multiple output rows. However, regexp_extract does the following:

• When used in the FROM clause, it adds multiple columns.
• When used in the SELECT clause, it seems to be creating a record (in one column).

I'd improve the docs myself, but I'm not sure about various things:

• Is regexp_extract the only table function with the above properties?
• Should we even consider it a table function if it's so different from all other table functions, or create a new category for it?
• Do other databases have something similar? Note that Cockroach and Databricks have a function with the same name, but it only returns one output value, so it doesn't have the above peculiarities.