Documentation clarification for mdl-progress

The [documentation for mdl-progress](http://www.getmdl.io/components/index.html#loading-section/progress) seems a bit inconsistent:
- The introductory samples use both `mdl-progress mdl-js-progress`.
- The "to include an MDL progress component" section demonstrates adding only `mdl-js-progress`, and then declares it ready for use.
- The next set of examples (text only, no actual component) use only `mdl-js-progress`.
- The "configuration options" section mentions only `mdl-js-progress`. There's no mention of `mdl-progress` here.

Empirically, it seems that I need both classes in order to get a working MDL progress component. Does that sound right?

The MDL spinner, documented on the same page, always uses `mdl-spinner mdl-js-spinner` in every example, and both classes are documented in the "configuration options" at the end.