Added message example for return-in-init (#6245)

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

Author: matusvalo

doc/data/messages/r/return-in-init/bad.py

@@ -0,0 +1,4 @@
+class Sum:
+
+    def __init__(self, a, b):  # [return-in-init]
+        return a + b

doc/data/messages/r/return-in-init/good.py

@@ -0,0 +1,4 @@
+class Sum:
+
+    def __init__(self, a, b) -> None:
+        self.result = a + b

doc/data/messages/r/return-in-init/related.rst

@@ -0,0 +1 @@
+- `__init__ method documentation <https://docs.python.org/3/reference/datamodel.html#object.__init__>`_