Add component classes to docs for child widgets?

[TomJGooding]

I spotted that the docs for Tabs has no mention of the underline--bar component class.

After a quick search I found a number of other widgets which say "This widget has no component classes" but where its child widgets do. I think this should be explained in the docs?

• docs/widgets/progress_bar.md -- see Bar Component Classes section?
• docs/widgets/tabbed_content.md -- see Underline (Tabs)?
• docs/widgets/markdown_viewer.md -- see Markdown and MarkDownTable Content?
• docs/widgets/tabs.md -- see Underline?
• docs/widgets/radioset.md -- see RadioButton?