[2.2.x] Refs #30083 -- Added a warning about performing queries in pre/post_init receivers.

Thanks Carlton Gibson the review.

Backport of fc1182af01c391ce33d7fcf51c756829c6a11d5b from master

docs/ref/signals.txt

@@ -106,6 +106,12 @@ Arguments sent with this signal:
         ``_state.db`` is ``None`` and cannot be used to check an ``instance``
         database.
 
+.. warning::
+
+    For performance reasons, you shouldn't perform queries in receivers of
+    ``pre_init`` or ``post_init`` signals because they would be executed for
+    each instance returned during queryset iteration.
+
 ``pre_save``
 ------------