container/heap: docs do not clearly state that element 0 is minimum element

<pre>I believe the implementation of container/heap implies that the 0 element is always the
minimum element.  Thus you can always peek at the minimum element without actually
popping it by simply looking at h[0].  But the docs don't actually state this, and there
is no Top method that lets you look at the minimum element without removing it.</pre>
