pytest-dev#2692: Document setup/teardown behaviour when using unittes…

…t-based suites

doc/en/unittest.rst

@@ -21,7 +21,7 @@ their ``test`` methods in ``test_*.py`` or ``*_test.py`` files.
 Almost all ``unittest`` features are supported:
 
 * ``@unittest.skip`` style decorators;
-* ``setUp/tearDown``;
+* ``setUp/tearDown`` (see :ref:`note regarding test phases` below);
 * ``setUpClass/tearDownClass()``;
 
 .. _`load_tests protocol`: https://docs.python.org/3/library/unittest.html#load-tests-protocol
@@ -233,3 +233,14 @@ was executed ahead of the ``test_method``.
     overwrite ``unittest.TestCase`` ``__call__`` or ``run``, they need to
     to overwrite ``debug`` in the same way  (this is also true for standard
     unittest).
+
+.. _note_regarding_test_phases:
+.. note::
+
+    Due to architectural differences between the two frameworks, setup and
+    teardown for ``unittest``-based tests is performed during the ``call`` phase
+    of testing instead of in ``pytest``'s standard ``setup`` and ``teardown``
+    stages. This can be important to understand in some situations, particularly
+    when reasoning about errors. For example, if a ``unittest``-based suite
+    exhibits errors during setup, ``pytest`` will report no errors during its
+    ``setup`` phase and will instead raise the error during ``call``.