Make ElementContentHandlers and DocumentContentHandlers fields public.

[orium]

This is helpful if you want to patch already created handlers with extra functionality. For instance, if you want to measure the time a handler takes to run you can do something like:

fn measure_element_time_handler(handlers: ElementContentHandlers<'_>) -> = ElementContentHandlers<'_> {
   let element_handler = self.element.take().map(|handler| {
       |element| {
           // measure start time here.
           let result = handler(element);
           // measure end time here.
           result
       }
   });

   ElementContentHandlers {
       element: element_handler,
       comments: self.comments.take(),
       text: self.text.take(),
   }
}

[inikulin]

We need doc comments for those and all the newly exposed API. Let's add #![warn(missing_docs)] to the crate's lib.rs and also use `RUSTFLAGS="-D warnings" to the CI builds (if we don't have it already) to fail on missing docs

[orium]

Will add documentation to those fields. However, we can't fail on missing documentation right because it is missing from a lot of places:

warning: `lol_html` (lib) generated 21 warnings

[inikulin]

Let's revisit those and suppress the warning where it's intentional or provide docs otherwise.