Update doc comments on service-connection-provider.ts (#13805)

Add some documentation to make the usage of the local/remote service id's more understandable.

contributed on behalf of STMicroelectronics

packages/core/src/browser/messaging/service-connection-provider.ts

@@ -20,15 +20,29 @@ import { ChannelMultiplexer } from '../../common/message-rpc/channel';
 import { Deferred } from '../../common/promise-util';
 import { ConnectionSource } from './connection-source';
 
+/**
+ * Service id for the local connection provider
+ */
 export const LocalConnectionProvider = Symbol('LocalConnectionProvider');
+/**
+ * Service id for the remote connection provider
+ */
 export const RemoteConnectionProvider = Symbol('RemoteConnectionProvider');
 
 export namespace ServiceConnectionProvider {
     export type ConnectionHandler = (path: String, channel: Channel) => void;
 }
 
 /**
- * This class manages the channels for remote services in the back end
+ * This class manages the channels for remote services in the back end.
+ *
+ * Since we have the ability to use a remote back end via SSH, we need to distinguish
+ * between two types of services: those that will be redirected to the remote back end
+ * and those which must remain in the local back end. For example the service that manages
+ * the remote ssh connections and port forwarding to the remote instance must remain local
+ * while e.g. the file system service will run in the remote back end. For each set
+ * of services, we will bind an instance of this class to {@linkcode LocalConnectionProvider}
+ * and {@linkcode RemoteConnectionProvider} respectively.
  */
 @injectable()
 export class ServiceConnectionProvider {