docs: correct loaded() - server-side only, not a client-side hook (#361)

Document that loaded() belongs to node_helper, not Module.
Add async/Promise support to the start() docs as the correct
way to handle asynchronous module initialization.

Author: KristjanESPERANTO

module-development/core-module-file.md

@@ -86,35 +86,32 @@ requiresVersion: "2.1.0",
 This method is called when a module gets instantiated. In most cases you do not
 need to subclass this method.
 
-### `loaded(callback)`
+### `start()`
 
-_Introduced in version: 2.1.1._
+This method is called when all modules are loaded and the system is ready to
+boot up. Keep in mind that the dom object for the module is not yet created. The
+start method is a perfect place to define any additional module properties.
 
-This method is called when a module is loaded. Subsequent modules in the config
-are not yet loaded. The `callback` function MUST be called when the module is
-done loading. In most cases you do not need to subclass this method.
+This method can be `async` or return a `Promise`. The system will wait for all
+modules' `start()` promises to settle (via `Promise.allSettled`) before
+proceeding to create the DOM and call `getDom()`. This makes it the right place
+for any asynchronous initialization, such as waiting for Web Components to be
+registered.
 
 **Example:**
 
 ```js
-loaded: function(callback) {
-	this.finishLoading();
-	Log.log(this.name + ' is loaded!');
-	callback();
+start: function() {
+	this.mySpecialProperty = "So much wow!";
+	Log.log(this.name + ' is started!');
 }
 ```
 
-### `start()`
-
-This method is called when all modules are loaded and the system is ready to
-boot up. Keep in mind that the dom object for the module is not yet created. The
-start method is a perfect place to define any additional module properties:
-
-**Example:**
+**Async example:**
 
 ```js
-start: function() {
-	this.mySpecialProperty = "So much wow!";
+async start() {
+	await customElements.whenDefined("my-custom-element");
 	Log.log(this.name + ' is started!');
 }
 ```

module-development/node-helper.md

@@ -90,6 +90,14 @@ requiresVersion: "2.1.0",
 This method is called when a node helper gets instantiated. In most cases you do
 not need to subclass this method.
 
+### `loaded()`
+
+_Introduced in version: 2.1.1._
+
+This method is called after the node helper is loaded and its `name` and `path`
+properties have been set, but before `start()` is called. In most cases you do
+not need to subclass this method.
+
 ### `start()`
 
 This method is called when all node helpers are loaded and the system is ready