Noplace in the documentation actually defined what these variables
contain.  Define them as lists of arguments for LOAD, and improve
that command's documentation a bit.

Discussion: https://postgr.es/m/CAB-oJtxHVDc3H+Km3CjB9mY1VDzuyaVH_ZYSz7iXcRqCtb93Ew@mail.gmail.com