The latest version of the Mission Planner (1.0.55) has a nice little feature that you might not notice, so I thought I'd call it out. In the Configuration tab, where all the user-modifiable MAVLink parameters are, "tool tips" that explain what each does now appear when you hover over each parameter. Very handy for figuring out what does what and how to modify them.
The full list of MAVLink parameters is in the manual here, but now you don't need to look them up. It's right there in the Mission Planner. Another great update by Michael Oborne!