Clarify and tidy up KeywordLine Argument
Non-semantic change:
- Introduce the notion of Argument, which is relied on in the rest of the document, but not defined here formally.
Spec changes implied by this diff:
-
Forbid the arguments from containing form feeds, carriage returns, and vertical tabs.
-
Forbid trailing whitespace after the last argument. I'm not sure whether this is correct, but it seems anomalous to permit it for only for keyword lines with arguments, and not anywhere else.