manual: convert-* interface, optimizing translators overview optimized

- (Wrapper generators): Add comment about the 30-90% figure.
- Reword `Pointers' to be shorter/clearer.
- Promote `Optimizing Type Translators' to a full section, a few other
  things here, most conspicuously expanding on the note about expand-*
  method definition time.
- (Tutorial-Conclusion): Not a first draft anymore.
- Document convert-to-foreign, convert-from-foreign, and
  free-converted-object.