Explain why older changes are hidden more clearly

Summary:
Fixes T9920. When hiding changes, tell users why so they can learn the comment rule (usually, "Changes from before your most recent comment are hidden."; sometimes they're hidden for pagination reasons).

Also use "Show Older Comments" instead of "Show older comments." for the action since I think it's a little more consistent to use title case for links/actions?

Test Plan:

• Viewed a task with a lot of comments, saw a "most recent comment" element.
• Artificially set page size to 3, saw a "lots of changes" hide.
• Grepped for removed string.
• Clicked both "show older stuff" links.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9920

Differential Revision: https://secure.phabricator.com/D14719