Add onCustomEditor to documented activation events

src/vs/workbench/services/extensions/common/extensionsRegistry.ts

@@ -289,6 +289,11 @@ export const schema: IJSONSchema = {
 						body: 'onUri',
 						description: nls.localize('vscode.extension.activationEvents.onUri', 'An activation event emitted whenever a system-wide Uri directed towards this extension is open.'),
 					},
+					{
+						label: 'onCustomEditor',
+						body: 'onCustomEditor:${9:viewType}',
+						description: nls.localize('vscode.extension.activationEvents.onCustomEditor', 'An activation event emitted whenever the specified custom editor becomes visible.'),
+					},
 					{
 						label: '*',
 						description: nls.localize('vscode.extension.activationEvents.star', 'An activation event emitted on VS Code startup. To ensure a great end user experience, please use this activation event in your extension only when no other activation events combination works in your use-case.'),