Part of #7898.

In preparing Web/API docs for Markdown conversion we'd like to remove occurrences of <div class="hidden">. There are two main usages of this pattern:

1. to include comments for editors, like the comments attached to interactive examples
2. to hide code blocks that the author wants to participate in live samples, but doesn't want the reader to see

This issue to about removing instances of the first category. The fix in this case is often just to delete the content, although there might be some exceptions.

For an example of this pattern:

Sometimes this happens when an author has set hidden directly on a <pre> block that's part of a live sample. In this case we should keep the hidden class.