intra-rustdoc links: Consider supporting `Self`

[crumblingstatue]

Self is commonly used to avoid having to repeat the same type name over and over again. This makes renaming the type in the future, etc. easier, and DRY code is generally desirable.

Currently, the following produces a link with the literal text self::new instead of a correct link:

/// Use [`new`] to create a new instance.
///
/// [`new`]: Self::new
pub struct Foo;

impl Foo {
	/// Creates a new instance.
	pub fn new() -> Self {
		unimplemented!()
	}
}

Ref #43466