DEV: introduces documentation for chat (#19772)

Note this commit also slightly changes internal API: channel instead of getChannel and updateCurrentUserChannelNotificationsSettings instead of updateCurrentUserChatChannelNotificationsSettings.

Also destroyChannel takes a second param which is the name confirmation instead of an optional object containing this confirmation. This is to enforce the fact that it's required.

In the future a top level jsdoc config file could be used instead of the hack tempfile, but while it's only an experiment for chat, it's probably good enough.

.jsdoc

@@ -0,0 +1,7 @@
+// jsdoc doesn't accept paths starting with _ (which is the case on github runners)
+// so we need to alter the default config
+{
+  "source": {
+    "excludePattern": ""
+  }
+}