Add links to cairo API documentation in the xml doc comments

[gfoidl]

E.g.

CairoSharp/source/CairoSharp/CairoContext.cs

Lines 103 to 113 in 8ab5a9a

	/// <summary>
	/// Checks whether an error has previously occurred for this context.
	/// </summary>
	public Status Status
	{
	get
	{
	this.CheckDisposed();
	return cairo_status(this.Handle);
	}
	}

it's well documented, but ideally there could be a link to https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-status as well.

---

Very low priority, and there are lots of APIs in cairo (I should have done this from the beginning).