Clarify implicit_reexport with strict mode

Author: ahmojo

docs/source/config_file.rst

@@ -837,6 +837,10 @@ section of the command line docs.
     the item is imported using from-as or is included in ``__all__``. Note that mypy
     treats stub files as if this is always disabled. For example:
 
+    Although this option is enabled by default, :confval:`strict` enables
+    :option:`mypy --no-implicit-reexport`. Set ``implicit_reexport = true``
+    explicitly if you want to keep implicit reexports when using strict mode.
+
     .. code-block:: python
 
        # This won't re-export the value