[Ada] Document Stubs_Subdir in gnattest section on user guide

2018-05-31  Fedor Rybin  <frybin@adacore.com>

gcc/ada/

	* doc/gnat_ugn/gnat_utility_programs.rst: Document Stubs_Subdir in
	gnattest section on user guide.

From-SVN: r261015

gcc/ada/ChangeLog

+2018-05-31  Fedor Rybin  <frybin@adacore.com>
+
+	* doc/gnat_ugn/gnat_utility_programs.rst: Document Stubs_Subdir in
+	gnattest section on user guide.
+
 2018-05-31  Ed Schonberg  <schonberg@adacore.com>
 
 	* sem_res.adb (Resolve_String_Literal): If the type is a string type

gcc/ada/doc/gnat_ugn/gnat_utility_programs.rst

@@ -4788,6 +4788,9 @@ Alternatively, you may run the script using the following command line:
        is used to select the same output mode as with the ``--tests-dir`` option.
        This attribute cannot be used together with ``Subdir`` or ``Tests_Root``.
 
+  * ``Stubs_Dir``
+       is used to select the same output mode as with the ``--stubs-dir`` option.
+
   * ``Harness_Dir``
        is used to specify the directory in which to place harness packages and project
        file for the test driver, otherwise specified by ``--harness-dir``.