Mention that `pytest_plugins` should not be used as module name

changelog/3899.doc.rst

@@ -0,0 +1 @@
+Add note to ``plugins.rst`` that ``pytest_plugins`` should not be used as a name for a user module containing plugins.

doc/en/plugins.rst

@@ -86,6 +86,11 @@ which will import the specified module as a ``pytest`` plugin.
     :ref:`full explanation <requiring plugins in non-root conftests>`
     in the Writing plugins section.
 
+.. note::
+   The name ``pytest_plugins`` is reserved and should not be used as a
+   name for a custom plugin module.
+
+
 .. _`findpluginname`:
 
 Finding out which plugins are active