docs: restore accidentally removed subtest output explanation

Author: emilymamcarz

doc/en/how-to/subtests.rst

@@ -61,6 +61,14 @@ Each assertion failure or error is caught by the context manager and reported in
     FAILED test_subtest.py::test - contains 2 failed subtests
     3 failed, 3 subtests passed in 0.12s
 
+In the output above:
+
+* Subtest failures are reported as ``SUBFAILED``.
+* Subtests are reported first and the "top-level" test is reported at the end on its own.
+
+Note that it is possible to use ``subtests`` multiple times in the same test, or even mix and match with normal assertions
+outside the ``subtests.test`` block:
+
 .. code-block:: python
 
     def test(subtests):