More documentation updates for LATERAL.

Extend xfunc.sgml's discussion of set-returning functions to show an
example of using LATERAL, and recommend that over putting SRFs in the
targetlist.

In passing, reword func.sgml's section on set-returning functions so
that it doesn't claim that the functions listed therein are all the
built-in set-returning functions.  That hasn't been true for a long
time, and trying to make it so doesn't seem like it would be an
improvement.  (Perhaps we should rename that section?)

Both per suggestions from Merlin Moncure.