[path_parsing] Add docs for public APIs

[stuartmorgan-g]

When importing path_parsing, I had to add // ignores 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.)

[mozammal-hossain]

@stuartmorgan-g hi, I've created a PR on this issue. could you assign this to me?

[stuartmorgan-g]

@mozammal-hossain I don't generally assign issues to non-members, as they cannot remove their assignment if (as very frequently happens) they don't actually complete the process of getting to a landable PR.