Added missing-return-type-doc message example

Author: matusvalo

doc/data/messages/m/missing-return-type-doc/bad.py

@@ -0,0 +1,7 @@
+def integer_sum(a, b):  # [missing-return-type-doc]
+    """Returns sum of two integers
+    :param a: first integer
+    :type a: int
+    :return: sum of parameters a and b
+    """
+    return a + b

doc/data/messages/m/missing-return-type-doc/good.py

@@ -0,0 +1,10 @@
+def integer_sum(a, b):
+    """Returns sum of two integers
+    :param a: first integer
+    :type a: int
+    :param b: second integer
+    :type b: int
+    :return: sum of parameters a and b
+    :rtype: int
+    """
+    return a + b