Added note to writing_plugins.rst (#7896)

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

doc/en/reference.rst

@@ -688,6 +688,10 @@ items, delete or otherwise amend the test items:
 
 .. autofunction:: pytest_collection_modifyitems
 
+.. note::
+    If this hook is implemented in ``conftest.py`` files, it always receives all collected items, not only those
+    under the ``conftest.py`` where it is implemented.
+   
 .. autofunction:: pytest_collection_finish
 
 Test running (runtest) hooks

doc/en/writing_plugins.rst

@@ -107,6 +107,10 @@ Here is how you might run it::
 
     See also: :ref:`pythonpath`.
 
+.. note::
+    Some hooks should be implemented only in plugins or conftest.py files situated at the
+    tests root directory due to how pytest discovers plugins during startup,
+    see the documentation of each hook for details.
 
 Writing your own plugin
 -----------------------