docs: Add note about SIMPLE tests datadir

There is an extensive description on how INSTRUMENTED test datadir
hierarchy looks like, but it might be useful to mention SIMPLE test as
well to see the difference.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

docs/source/WritingTests.rst

@@ -208,6 +208,10 @@ to be accessed from up to three sources:
    ``MyTest.test_foo``, with variant ``debug-ffff``, the data directory path
    will be ``/home/user/test.py.data/MyTest.test_foo/debug-ffff/``.
 
+.. note:: Unlike INSTRUMENTED tests, SIMPLE tests only define ``file``
+          and ``variant`` data_dirs, therefore the most-specific data-dir
+          might look like ``/bin/echo.data/debug-ffff/``.
+
 Avocado looks for data files in the order defined at
 :attr:`DATA_SOURCES <avocado.core.test.TestData.DATA_SOURCES>`, which are
 from most specific one, to most generic one.  That means that, if a variant