Improve documentation of struct ParseState.

I got annoyed about how some fields of ParseState were documented in the
struct's block comment and some weren't; not all of the latter are trivial.
Fix that.  Also reorder a couple of fields that seem to have been placed
rather randomly, or maybe with an idea of avoiding padding space; but there
are never so many ParseStates in existence at one time that we ought to
value pad space over readability.