Previously it was unclear exactly how ROWS FROM behaved and how to cast
the data types of columns returned by FROM functions.  Also document
that only non-OUT record functions can have their columns cast to data
types.

Reported-by: guyren@gmail.com

Discussion: https://postgr.es/m/158638264419.662.2482095087061084020@wrigleys.postgresql.org

Backpatch-through: 9.5