Document ~ and $ for leaderboards page

[kdecks]

~ will submit the largest value of multiple lookups.

example: 0x0f3336*~0xM0f3314$0x0f32d6*~0xM0f32b4$0x0f3306*~0xM0f32e4$0x0f3366*~0xM0f3344 for http://retroachievements.org/leaderboardinfo.php?i=2282

I thought that maybe I/we could use this as a to-do list of things that need documentation, maybe. That anyone could work on.