Update Javadoc for setMaxSpeed(). Addresses BUKKIT-4242

The default maxSpeed has changed in vanilla from 0.4 to 0.35 - this commit updates the Javadocs to reflect that fact.
Implement get/set damage method for boats. Adds BUKKIT-5325

This brings the same functionality to boats that currently exists only for minecarts.
Deprecate occupiedDeceleration

Due to changes in how Minecraft handles boats, this value is no longer
constant, but instead dynamically calculated based on other variables.