Before commit d3cc37f1, an inheritance parent
whose only children were temp tables of other sessions would end up
as a simple scan of the parent; but with that commit, we end up with
an Append node, per a report from Ashutosh Bapat.  Tweak the logic
so that we go back to the old way, and update the function header
comment for partitioning while we're at it.

Ashutosh Bapat, reviewed by Amit Langote and adjusted by me.

Discussion: http://postgr.es/m/CAFjFpReWJr1yTkHU=OqiMBmcYCMoSW3VPR39RBuQ_ovwDFBT5Q@mail.gmail.com