ExecAggTransReparent is underdocumented and badly named

I wondered at [1] why ExecAggTransReparent doesn't do what you'd
guess from the name, that is reparent a supplied R/W expanded object
under the aggcontext even if it's not there already.  I tried to
make it do that, and after I'd finished bandaging my wounds,
I wrote some commentary explaining why that won't work.  (Possibly
we knew this at some point, but if so we sure failed to provide
documentation about it.)

I think we should at least apply the attached commentary-only
patch.  I wonder though if we should change the name of this
function, and if so to what.  Maybe ExecAggCopyTransValue?

			regards, tom lane

[1] https://www.postgresql.org/message-id/2199319.1681662388%40sss.pgh.pa.us