Stabilize Document::to_dom_node

[silverpill]

Is your feature request related to a problem? Please describe.

Ability to post-process HTML documents is very useful, but currently requires using rustflags.

Describe the solution you'd like

I think it should be stabilized (it may remain feature-gated though).

Also, there seems to be an error in documentation:

ammonia/src/lib.rs

Lines 2831 to 2833 in 98c2eba

	/// let mut buf = Vec::new();
	/// serialize(&mut buf, &node, SerializeOpts::default())?;
	/// let output = String::from_utf8(buf)?;

serialize requires SerializableHandle:

let mut buf = Vec::new();
let handle = SerializableHandle::from(node);
serialize(&mut buf, &handle, SerializeOpts::default())?;

Describe alternatives you've considered

N/A