Adding a sample and explanation for doctests

docs/writing/tests.rst

@@ -37,9 +37,36 @@ Doctest
 -------
 
 The doctest module searches for pieces of text that look like interactive Python
-sessions, and then executes those sessions to verify that they work exactly as
+sessions in docstrings, and then executes those sessions to verify that they work exactly as
 shown.
 
+Doctests have a different use case than proper unit tests: they are usually less
+detailed and don't catch special cases or obscure regression bugs. They are
+useful as an expressive documentation of the main use cases of a module and
+its components. However, doctests should run automatically each time
+the full test suite runs.
+
+A simple doctest in a function:
+
+::
+
+    def square(x):
+        """Squares x.
+
+        >>> square(2)
+        4
+        >>> square(-2)
+        4
+        """
+
+        return x * x
+
+    if __name__ == '__main__':
+        import doctest
+        doctest.testmod()
+
+When running this module from the command line as in ``python module.py``, the doctests
+will run and complain if anything is not behaving as described in the docstrings.
 
 Tools
 :::::