style: Add docs about the lack of device in the stylesheet set.

Also add a few comments and similar about pending invalidation work, and avoid
passing a device in script, since the work it performs scanning the stylesheet
is just thrown away in the `flush_without_invalidations` call.

components/style/stylesheet_set.rs

@@ -106,6 +106,8 @@ where
     }
 
     /// Appends a new stylesheet to the current set.
+    ///
+    /// No device implies not computing invalidations.
     pub fn append_stylesheet(
         &mut self,
         device: Option<&Device>,