Document entrypoint for showing/hiding the Party Gauge

[lhearachel]

For clarity, the term "Party Gauge" refers to the following elements on the battle UI, which show at the start of the battle and whenever a trainer sends in a new Pokemon after a KO:

This is all foundational; the Party Gauge itself is my next target, and none of this code actually deals with graphics, but this documentation sets up a sketch of how messages are transferred between the battle back-end and what's actually rendered on the DS itself.

The indirection flow is essentially:

1. The battle back-end enqueues a message for publication to all interested parties. In PvE settings, this will just be the player's DS, but it includes all other connected players in Link-enabled battles.
2. Connected players consume the message, which contains a command ID that maps to a particular processing routine.
3. The processing routine directs control to an async task, which takes care of whatever needs to be displayed and waits for the result.