Work on javadoc for Brain, MemoryModule, Sensors and related.

[i509VCB]

These are used quite heavily in Piglins, Villagers and Hoglins. Discord discussions showed these are not very easy to understand.

A high level overview of what I know about these off head, please check yourself while writing javadoc of course:

• Brain: A data container for memories which store a value.
• MemoryModule: A registered memory entry in a brain.
• Sensor: Used to get context for the brain from a world.

[liach]

I can work on them. I think I mapped a lot of brain stuff back when they were added.