Add testonly parameter to index.bzl documentation

Author: sbarfurth

src/ng_project/index.bzl

@@ -18,6 +18,7 @@ def ng_project(
         angular_compiler_options: Dictionary of angular compiler options to provide to the compilation.
         assets: Assets needed for the Angular compilation
         tsconfig: The tsconfig to be used as a base configuration for the compilation.
+        testonly: Whether all targets generated by the macro are marked as testonly.
         **kwargs: Additional arguments passed along to the underlying ts_project.
     """