More comments describing the unordered internals.

And fix a couple of small mistakes in the existing comments.

[SVN r66555]

libs/unordered

@@ -1 +1 @@
-Subproject commit eb04f683510568f32a52fb06b07791f5cd42b746
+Subproject commit a49c76c69c6e5adf53a089110965854d3e1c0a2d