Mention the "inline comments" rule in the callout for "Large" diffs

Summary:
See PHI638. When a diff is large (between 100 and 1000 files), we collapse content by default unless a change also has inline comments.

This rule isn't explicitly explained anywhere. Although it's not really a critical rule, it fits easily enough into the UI callout.

Also render the UI callout in a slightly more modern way and avoid hsprintf().

Test Plan:

• Also, clicked the "Expand" link and saw everything expand properly.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19430