small refactor of MainButton

tgbotapi.webapps/src/jsMain/kotlin/dev/inmo/tgbotapi/webapps/MainButton.kt

@@ -22,6 +22,9 @@ external class MainButton {
     fun showProgress(leaveActive: Boolean = definedExternally): MainButton
     fun hideProgress(): MainButton
 
+    /**
+     * **This method argument do not accept `this` [WebApp] object**
+     */
     fun onClick(eventHandler: () -> Unit): MainButton
     fun offClick(eventHandler: () -> Unit): MainButton
 
@@ -36,7 +39,7 @@ data class MainButtonParams(
     val isVisible: Boolean? = null
 )
 
-@Deprecated(message="Use onClick without EventHandler")
+@Deprecated(message = "Use onClick without EventHandler")
 fun MainButton.onClick(eventHandler: EventHandler) = onClick {
     val that = js("this").unsafeCast<WebApp>()
     that.eventHandler()