The comment describing the string format was a lie.  Make it agree with
reality, add/improve some other comments, fix coding style for loops with
empty bodies.  Also add an Assert that we counted parameters correctly,
because the spread-out logic for that looks pretty fragile.

No actual bugs fixed here, so no need to back-patch.

Discussion: https://postgr.es/m/848B1649C8A6274AA527C4472CA11EDD5FC70CBE@G01JPEXMBYT02