Change the restrictions for using self.

Based on in office conversation the use of self to reference the class
has the potential benefit of handling renames in the event that the
class name needs to change. Additionally it makes for easier extraction
into a new class.
Could we avoid this debate by just removing "explicitly" from the guideline? Does "when you need to reference an object's class" or "when you need to invoke the `class` method" more clearly state what we want?