Changes regular comment to doc comment

[bluss]

This change is a change in semantics:

-///// ChunksLazy /////
+/// // ChunksLazy /////

If this would have been at the end of the file (no item after), it breaks the build.

[nikomatsakis]

I encountered a similar problem in librustc_data_structures:

-    ///////////////////////////////////////////////////////////////////////////
-    // Edge construction and queries
+    /// ////////////////////////////////////////////////////////////////////////
+    /// Edge construction and queries

[nrc]

dup of #652