[chore] Update doc on Option (#366)

The doc on Option mentions that there is no concrete implementation of it,
but that is no longer the case as we've added several options to configure
the container.

This updates that part of the doc.

container.go

@@ -53,8 +53,7 @@ func (k key) String() string {
 	return k.t.String()
 }
 
-// Option configures a Container. It's included for future functionality;
-// currently, there are no concrete implementations.
+// Option configures a Container.
 type Option interface {
 	applyOption(*Container)
 }