Better documentation for row_number() combined with set returning functions

Hi

The current documentation of Postgresql sounds like row_number() over
() can be used to number the rows returned by postgres. However, that
doesn't work when the query also uses set returning functions such as
json_array_elements. In this case, row_number() will be the same for
every element of the set.

I suggest to add a paragraph to the description of row_number() that
states that this is the behaviour and maybe also reference the "with
ordinality" feature that can be used instead.

Erik