add support for internal getter and setter functions

Adding a `#[doc(hidden)]` where `#[func]` is present hides the function from Godot's documentation which is great.

This seems to be undocumented which caused me to spend some time trying to hide an internal/hidden getter/setter function when a simple `#[doc(hidden)]` attribute would have been enough.

Would be nice if it was added to the docs so anyone else doesn't waste time like I did.

Start by locating the documentation for #[func] and the existing explanation of #[doc(hidden)]. Add a note explaining that #[doc(hidden)] can hide internal getter and setter functions from Godot's documentation, then verify the resulting documentation clearly covers this use case.

Written by the indexing model from the issue text.