Update docs for async changes in 27dbfd7
Update docs for async changes in 27dbfd7

- Update (and refactor) example for Usage;
- Update (and refactor) example for Retext(parser);
- Update description of API: Retext(parser);
- Rename Retext.prototype.parse(source) >
  Retext.prototype.parse(source, callback);
- Update description of API:
  Retext.prototype.parse(source, callback).

wooorm/retext,azu/retext