Started documenting battle messages

Added documentation for the following battle messages: request, move,
switch, -damage/-heal, -boost/-unboost, faint, cant.

Other messages will come next, followed perhaps by how to send moves.

Also, changed most bold headers to h4 headers so that the hierarchy for
battle messages could be more obvious.
Merge pull request #1112 from ascriptmaster/master

Started documentation on battle messages