Add comment specifying the iteration order of flat_set/flat_tree.

Iteration order was implied but never stated before. We need to rely on
it for correctness. eg. crrev.com/c/2398449

Change-Id: I2050dffb44da68950261db319981537178fea2e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2407622
Auto-Submit: Glen Robertson <glenrob@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809781}

base/containers/flat_tree.h

@@ -36,8 +36,8 @@ struct IsTransparentCompare<T, void_t<typename T::is_transparent>>
 
 // Implementation -------------------------------------------------------------
 
-// Implementation of a sorted vector for backing flat_set and flat_map. Do not
-// use directly.
+// Implementation for the sorted associative flat_set and flat_map using a
+// sorted vector as the backing store. Do not use directly.
 //
 // The use of "value" in this is like std::map uses, meaning it's the thing
 // contained (in the case of map it's a <Kay, Mapped> pair). The Key is how
@@ -156,6 +156,9 @@ class flat_tree {
 
   // --------------------------------------------------------------------------
   // Iterators.
+  //
+  // Iterators follow the ordering defined by the key comparator used in
+  // construction of the flat_tree.
 
   iterator begin();
   const_iterator begin() const;