R1:5d5fd9e24177

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"…