Clean up intro.
Whitespace.
Stubbing out the caching concept.

There's not much to do here, as HTML doesn't specify how a browser's
internal cache can/should work. We're limited to making recommendations
about how integrity metadata could be useful, and pointing out risks of
the näive approach of simply grabbing data and reusing it wildly.

This patch moves caching out to a top-level TOC entry, and moves the two
security risk sections out of 'security considerations' and into the new
block for clarity.
Regenerating draft.