Properly disallow async Player#chat (#8123)

Clarify asynchronous status of AsyncChatEvent

patches/api/Adventure.patch

@@ -512,6 +512,16 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +
 +/**
 + * An event fired when a {@link Player} sends a chat message to the server.
++ * <p>
++ * This event will sometimes fire synchronously, depending on how it was
++ * triggered.
++ * <p>
++ * If a player is the direct cause of this event by an incoming packet, this
++ * event will be asynchronous. If a plugin triggers this event by compelling a
++ * player to chat, this event will be synchronous.
++ * <p>
++ * Care should be taken to check {@link #isAsynchronous()} and treat the event
++ * appropriately.
 + */
 +public final class AsyncChatEvent extends AbstractChatEvent {
 +    private static final HandlerList HANDLERS = new HandlerList();