tsdb: Complete chunk format documentation
This also tweaks and fixes a few things done previously.
@@ -27,8 +27,6 @@ const ()
// HistogramChunk holds encoded sample data for a sparse, high-resolution
// histogram.
//
-// TODO(beorn7): Document the layout of chunk metadata.
-//
// Each sample has multiple "fields", stored in the following way (raw = store
// number directly, delta = store delta to the previous number, dod = store
// delta of the delta to the previous number, xor = what we do for regular
