(cmake_xmllint) include new args in docs

Author: MatthijsBurgh

ament_cmake_xmllint/doc/index.rst

@@ -29,13 +29,30 @@ How to run the check from within a CMake ament package as part of the tests?
     find_package(ament_cmake REQUIRED)
     if(BUILD_TESTING)
       find_package(ament_cmake_xmllint REQUIRED)
-      ament_xmllint()
+      ament_xmllint(
+        # PATHS .
+        # EXCLUDE specific_file.xml
+        # EXTENSIONS xml urdf xacro
+      )
     endif()
 
 When running multiple linters as part of the CMake tests the documentation of
 the package `ament_lint_auto <https://github.com/ament/ament_lint>`_ might
 contain some useful information.
 
+When you want to customize the extensions of the files to be checked, while using `ament_lint_auto`, you can use the following code snippet:
+
+``CMakeLists.txt``:
+
+.. code:: cmake
+
+    find_package(ament_cmake REQUIRED)
+    if(BUILD_TESTING)
+      find_package(ament_lint_auto REQUIRED)
+      set(ament_cmake_xmllint_EXTENSIONS "xml urdf xacro")
+      ament_lint_auto_find_test_dependencies()
+    endif()
+
 The documentation of the package `ament_cmake_test
 <https://github.com/ament/ament_cmake>`_ provides more information on testing
 in CMake ament packages.