Rename ExecAggTransReparent, and improve its documentation.

Rename ExecAggTransReparent, and improve its documentation.

The name of this function suggests that it ought to reparent R/W
expanded objects to be children of the persistent aggcontext, instead
of copying them.  In fact it does no such thing, and if you try to
make it do so you will see multiple regression failures.  Rename it
to the less-misleading ExecAggCopyTransValue, and add commentary
about why that attractive-sounding optimization won't work.  Also
adjust comments at call sites, some of which were describing logic
that has since been moved into ExecAggCopyTransValue.

Discussion: https://postgr.es/m/3004282.1681930251@sss.pgh.pa.us