Misc stuff for main commit

Simplify explain output

Remove obsolete comment on explain output

Justify previous subselect.c comments

wCTE plan list comments updated

Remove over-explanation of param plans in executor

Consolidate remarks on child plans

Don't append resultrelation where unnecessary

Remove comment regarding setrefs.c + excluded RTE

Much better explanation of auxiliary plan concept in comments

Provisional: refactor costing stuff
EXCLUDED stuff

Better comments on contract between EPQ + ExcludedExpr

Not RTE, INNER_VAR varno