Parser: document or update BitSet usage.

[evacchi]

Parser allows to pass in a BitSet instance in its constructor to select the sections that should be parsed.

However, there is no documentation on what the BitSet represents, and how it should be built. Either document how to build a proper BitSet, or wrap it in an explicit builder.

Besides, there does not seem to be usages in the codebase, so it might be a candidate for removal (or at least, to make it package-private)

e.g.

class ParserOptions {
    BitSet includedSections;
    ParserOptions withImportSection() {
         includedSections.set(SectionId.IMPORT);
         return this;
   }
   // ... etc
}
class Parser {
    public Parser(ParserOptions opts) {