Document calendar summary icons

Summary:
Fixes T11809. Ref

• Explicitly document the summary icon hints -- I don't think these are too hard to figure out (and maybe this stuff should just go in the tooltips) but we can start here.
• Use color + shape to distinguish between "cancelled" and "declined", not just color (for users with vision accessibility issues).
• Translate a "minute(s)" string into sensible English.
• Use RSVP status on the month view green circle thing.

Test Plan:

• Read docs.
• Looked at month view.
• Read reminder mail.
• Viewed month view mobile view.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11809

Differential Revision: https://secure.phabricator.com/D16872