Commit c6c919dc (Remove all occurrences of the obsolete
'parameters-extra' option) removed

	parameters-extra = true

because after 5c980caf (Drop the 'parameters-extra' option and always
forward extra parameters) "$@" is always implicitly added to argv.

Clarify the comment that was explaining what `parameters-extra = true`
was doing, since it was not updated during parameters-extra removal.