more plugin doc updates (#7767)

plugin/plugin.go

@@ -0,0 +1,12 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+// The plugin package defines the primary interfaces for interacting with a Mattermost server: the
+// API and the hook interfaces.
+//
+// The API interface is used to perform actions. The Hook interface is used to respond to actions.
+//
+// Plugins should define a type that implements some of the methods from the Hook interface, then
+// pass an instance of that object into the rpcplugin package's Main function (See the HelloWorld
+// example.).
+package plugin