Added useless-return message example (#6247)

Author: matusvalo

doc/data/messages/u/useless-return/bad.py

@@ -0,0 +1,6 @@
+import sys
+
+
+def print_python_version():  # [useless-return]
+    print(sys.version)
+    return None

doc/data/messages/u/useless-return/good.py

@@ -0,0 +1,5 @@
+import sys
+
+
+def print_python_version():
+    print(sys.version)