docs($urlMatcherFactory): document param type
docs($state): improve stateProvider.state() docs
fix($urlMatcherFactory): made path params default value "" for backwards compat

- 0.2.x will continue to treat path params as optional, coerced to an empty string ""
- 0.3.0 will treat path parameters without default values as required, and thus the transition without required params will fail.

closes #1476