Documentation: kunit: Update description of --alltests option

kunit_tool's --alltests option was changed in commit
980ac3ad05 ("kunit: tool: rename all_test_uml.config, use it for --alltests")
to use a manually curated list of architecture-indpendent Kconfig
options, rather than attempting to use make allyesconfig on UML, which
was broken.

Update the kunit_tool documentation to reflect the new behaviour of
--alltests.

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kunit/run_wrapper.rst

@@ -251,14 +251,15 @@ command line arguments:
   compiling a kernel (using ``build`` or ``run`` commands). For example:
   to enable compiler warnings, we can pass ``--make_options W=1``.
 
-- ``--alltests``: Builds a UML kernel with all config options enabled
-  using ``make allyesconfig``. This allows us to run as many tests as
-  possible.
+- ``--alltests``: Enable a predefined set of options in order to build
+  as many tests as possible.
 
-  .. note:: It is slow and prone to breakage as new options are
-            added or modified. Instead, enable all tests
-            which have satisfied dependencies by adding
-            ``CONFIG_KUNIT_ALL_TESTS=y`` to your ``.kunitconfig``.
+  .. note:: The list of enabled options can be found in
+            ``tools/testing/kunit/configs/all_tests.config``.
+
+            If you only want to enable all tests with otherwise satisfied
+            dependencies, instead add ``CONFIG_KUNIT_ALL_TESTS=y`` to your
+            ``.kunitconfig``.
 
 - ``--kunitconfig``: Specifies the path or the directory of the ``.kunitconfig``
   file. For example: