Added missing-return-doc message example (#6342)

Author: matusvalo

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

@@ -0,0 +1,6 @@
+def integer_sum(a: int, b: int):  # [missing-return-doc]
+    """Returns sum of two integers
+    :param a: first integer
+    :param b: second integer
+    """
+    return a + b

doc/data/messages/m/missing-return-doc/details.rst

@@ -0,0 +1 @@
+This message is raised only when parameter ``accept-no-return-doc`` is set to ``no``.

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

@@ -0,0 +1,7 @@
+def integer_sum(a: int, b: int) -> int:
+    """Returns sum of two integers
+    :param a: first integer
+    :param b: second integer
+    :return: sum of parameters a and b
+    """
+    return a + b

doc/data/messages/m/missing-return-doc/pylintrc

@@ -0,0 +1,5 @@
+[master]
+load-plugins=pylint.extensions.docparams
+
+[Parameter_documentation]
+accept-no-return-doc=no