Add doc comments for cosmetic filter structs/methods

[antonok-edm]

As per brave/brave-core#3303 (comment) I wanted to make sure it's very clear how the cosmetic filtering interface is to be used.

[antonok-edm]

Should be as simple as this:

let resources = engine.hostname_cosmetic_resources("github.com");

let stylesheet = engine.class_id_stylesheet(&["new_class1", "new_class2"], &["new_id1"], resources.exceptions.clone());

I'm noticing now that the signature for Engine::hostname_cosmetic_resources should just be updated to take exceptions by reference instead of by value. Once that's done, you'll even be able to remove the .clone() call.

let resources = engine.hostname_cosmetic_resources("github.com");

let stylesheet = engine.class_id_stylesheet(&["new_class1", "new_class2"], &["new_id1"], &resources.exceptions);