[path_parsing] Add docs for public APIs

When importing `path_parsing`, I had to add `// ignore`s for public_member_api_docs, because a lot of public APIs were undocumented, and figuring out all the APIs and documenting them was out of scope for the import. We should fix that and remove the `// ignore` so that we are following the best practices that our `analysis_options` are intended to enforce.

(This is also a `pub.dev` score demerit, and it's nice for packages under the `flutter.dev` publisher to set a good example in terms of following the practices we want the ecosystem to adopt.)