Doc: improve documentation of new SRF-in-tlist behavior.

Doc: improve documentation of new SRF-in-tlist behavior.

Correct a misstatement about how things used to work: we did allow nested
SRFs before, as long as no function had more than one set-returning input.

Also, attempt to document the fact that the new implementation changes the
behavior for SRFs within conditional constructs (eg CASE): the conditional
construct no longer gates whether the SRF is run, and thus cannot affect
the number of rows emitted.  We might want to change this behavior, but
first it behooves us to see if we can explain it.

Minor other wordsmithing on what I wrote yesterday, too.

Discussion: https://postgr.es/m/20170118214702.54b2mdbxce5piwv5@alap3.anarazel.de