Add a link to the MDN page for data attributes in the documentation f…

…or `WebHandle` (#86)

Not all people reading the documentation will know what a data attribute is (I didn't), and may assume that this just means a regular attribute.

src/web.rs

@@ -9,12 +9,14 @@
 #[non_exhaustive]
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct WebHandle {
-    /// An ID value inserted into the data attributes of the canvas element as '`raw-handle`'.
+    /// An ID value inserted into the [data attributes] of the canvas element as '`raw-handle`'.
     ///
     /// When accessing from JS, the attribute will automatically be called `rawHandle`.
     ///
     /// Each canvas created by the windowing system should be assigned their own unique ID.
     /// 0 should be reserved for invalid / null IDs.
+    ///
+    /// [data attributes]: https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/data-*
     pub id: u32,
 }