For the `DependencyMatcher`:
* Fix on_match callback so that it is called once per matched pattern
* Fix results so that patterns with empty match lists are not returned
* Implement new API for {Phrase}Matcher.add (backwards-compatible)
* Update docs
* Also update DependencyMatcher.add
* Update internals
* Rewrite tests to use new API
* Add basic check for common mistake
Raise error with suggestion if user likely passed in a pattern instead of a list of patterns
* Fix typo [ci skip]
<!--- Provide a general summary of your changes in the title. -->
## Description
* tidy up and adjust Cython code to code style
* improve docstrings and make calling `help()` nicer
* add URLs to new docs pages to docstrings wherever possible, mostly to user-facing objects
* fix various typos and inconsistencies in docs
### Types of change
enhancement, docs
## Checklist
<!--- Before you submit the PR, go over this checklist and make sure you can
tick off all the boxes. [] -> [x] -->
- [x] I have submitted the spaCy Contributor Agreement.
- [x] I ran the tests, and all new and existing tests passed.
- [x] My changes don't require a change to the documentation, or if they do, I've added all required information.
