Hide the (Offline)AudioContext::base() method from the public API

It is not necessary to expose it because the BaseAudioContext methods
are available on the object itself, plus it causes issues for the
`set_onstatechange` handler, see b0b03a6

Author: orottier

src/context/base.rs

@@ -21,6 +21,7 @@ use crate::{node, AudioListener};
 #[allow(clippy::module_name_repetitions)]
 pub trait BaseAudioContext {
     /// Returns the [`BaseAudioContext`] concrete type associated with this `AudioContext`
+    #[doc(hidden)] // we'd rather not expose the ConcreteBaseAudioContext
     fn base(&self) -> &ConcreteBaseAudioContext;
 
     /// Construct a new pair of [`AudioNode`] and [`AudioProcessor`]