Update docs: Mention two methods which can let you know if your code is running from within a pytest run

doc/en/example/simple.rst

@@ -405,7 +405,9 @@ Detect if running from within a pytest run
 Usually it is a bad idea to make application code
 behave differently if called from a test.  But if you
 absolutely must find out if your application code is
-running from a test you can do something like this:
+running from a test, you can follow one of the two ways listed below.
+
+This is a simple way to do it:
 
 .. code-block:: python
 
@@ -421,6 +423,51 @@ running from a test you can do something like this:
         # called "normally"
         ...
 
+This works great, but you should be aware that there is an issue with it. If there is a pytest import anywhere
+in your code flow, ``"pytest" in sys.modules`` will return True even if your code is not actually running from within a pytest run.
+
+.. code-block:: python
+
+    import sys
+
+    import pytest  # unused anywhere in your code
+
+
+    def is_called_from_test_by_pytest():
+        return "pytest" in sys.modules
+
+
+    # This method above will return True even if your code is not actually running from within a pytest run
+    # as there is a pytest import in your code flow
+
+This is a bit long but a robust way to do it:
+
+.. code-block:: python
+
+    # content of your_module.py
+
+
+    _called_from_test_by_pytest = False
+
+.. code-block:: python
+
+    # content of conftest.py
+
+
+    def pytest_configure(config):
+        your_module._called_from_test_by_pytest = True
+
+and then check for the ``your_module._called_from_test_by_pytest`` flag:
+
+.. code-block:: python
+
+    if your_module._called_from_test_by_pytest:
+        # called from within a test run
+        ...
+    else:
+        # called "normally"
+        ...
+
 accordingly in your application.
 
 Adding info to test report header