Document possible errors for Web

src/lib.rs

@@ -321,6 +321,8 @@ impl Surface {
     ///
     /// - On X11, the window must be visible.
     /// - On macOS, Redox and Wayland, this function is unimplemented.
+    /// - On Web, this will fail if the content was supplied by
+    ///   a different origin depending on the sites CORS rules.
     pub fn fetch(&mut self) -> Result<Vec<u32>, SoftBufferError> {
         self.surface_impl.fetch()
     }

src/web.rs

@@ -126,6 +126,10 @@ impl WebImpl {
 /// Extension methods for the Wasm target on [`Surface`](crate::Surface).
 pub trait SurfaceExtWeb: Sized {
     /// Creates a new instance of this struct, using the provided [`HtmlCanvasElement`].
+    ///
+    /// # Errors
+    /// - If the canvas was already controlled by an `OffscreenCanvas`.
+    /// - If a another context then "2d" was already created for this canvas.
     fn from_canvas(canvas: HtmlCanvasElement) -> Result<Self, SoftBufferError>;
 }