Add static return types to phpdocs

Also made the Closure return consisten use \Closure and some small typos. Replaces #1706

> Allow for proper chaining of functions
> MyModel::where('..')->orderBy('x')->first() should return instance of
> MyModel again.
> Doesn't work in every IDE, but is described in phpDocumentor/phpDocumentor2#800
> It does work in phpStorm/NetBeans, and doesn't seem to have side-effects in other IDEs like Sublime