Documentation: kunit: Update description of --alltests option

kunit_tool's --alltests option was changed in commit
980ac3ad ("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>