Bug 1609662: part 14) Document some `ContentIterator*` methods. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D60255

--HG--
extra : moz-landing-system : lando

dom/base/ContentIterator.h

@@ -31,7 +31,13 @@ class ContentIteratorBase {
 
   NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS(ContentIteratorBase)
 
+  /**
+   * Allows to iterate over the inclusive descendants
+   * (https://dom.spec.whatwg.org/#concept-tree-inclusive-descendant) of
+   * aRoot.
+   */
   virtual nsresult Init(nsINode* aRoot);
+
   virtual nsresult Init(nsRange* aRange);
   virtual nsresult Init(nsINode* aStartContainer, uint32_t aStartOffset,
                         nsINode* aEndContainer, uint32_t aEndOffset);
@@ -174,7 +180,11 @@ class ContentSubtreeIterator final : public ContentIteratorBase {
   ContentSubtreeIterator& operator=(const ContentSubtreeIterator&) = delete;
   virtual ~ContentSubtreeIterator() = default;
 
+  /**
+   * Not supported.
+   */
   virtual nsresult Init(nsINode* aRoot) override;
+
   virtual nsresult Init(nsRange* aRange) override;
   virtual nsresult Init(nsINode* aStartContainer, uint32_t aStartOffset,
                         nsINode* aEndContainer, uint32_t aEndOffset) override;