doc: show functions returning record types and use of ROWS FROM

doc:  show functions returning record types and use of ROWS FROM

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