Properly disallow async Player#chat (#8123)

Clarify asynchronous status of AsyncChatEvent

patches/api/0006-Adventure.patch

@@ -497,7 +497,7 @@ new file mode 100644
 index 0000000000000000000000000000000000000000..4eada40b8abb1833ce623ccee0789555e370d024
 --- /dev/null
 +++ b/src/main/java/io/papermc/paper/event/player/AsyncChatEvent.java
-@@ -0,0 +1,37 @@
+@@ -0,0 +1,47 @@
 +package io.papermc.paper.event.player;
 +
 +import java.util.Set;
@@ -512,6 +512,16 @@ index 0000000000000000000000000000000000000000..4eada40b8abb1833ce623ccee0789555
 +
 +/**
 + * 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();