style: Document Gecko's traversal code.

Actually, I think most of the functions in DOMTraversalContext deserve a nice
comment, but that's probably fine as a followup, I want to land the require-docs
thing now.

components/style/gecko/traversal.rs

@@ -2,6 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+//! Gecko-specific bits for the styling DOM traversal.
+
 use atomic_refcell::AtomicRefCell;
 use context::{SharedStyleContext, StyleContext, ThreadLocalStyleContext};
 use data::ElementData;
@@ -9,11 +11,14 @@ use dom::{NodeInfo, TNode};
 use gecko::wrapper::{GeckoElement, GeckoNode};
 use traversal::{DomTraversal, PerLevelTraversalData, recalc_style_at};
 
+/// This is the simple struct that Gecko uses to encapsulate a DOM traversal for
+/// styling.
 pub struct RecalcStyleOnly {
     shared: SharedStyleContext,
 }
 
 impl RecalcStyleOnly {
+    /// Create a `RecalcStyleOnly` traversal from a `SharedStyleContext`.
     pub fn new(shared: SharedStyleContext) -> Self {
         RecalcStyleOnly {
             shared: shared,