Mark onDidChangeAuthenticationProviders as deprecated in proposed API

src/vs/vscode.proposed.d.ts

@@ -112,6 +112,7 @@ declare module 'vscode' {
 		export function registerAuthenticationProvider(provider: AuthenticationProvider): Disposable;
 
 		/**
+		 * @deprecated - getSession should now trigger extension activation.
 		 * Fires with the provider id that was registered or unregistered.
 		 */
 		export const onDidChangeAuthenticationProviders: Event<AuthenticationProvidersChangeEvent>;