Added missing-module-docstring message example (#6249)

Author: matusvalo

doc/data/messages/m/missing-module-docstring/bad.py

@@ -0,0 +1,5 @@
+import sys  # [missing-module-docstring]
+
+
+def print_python_version():
+    print(sys.version)

doc/data/messages/m/missing-module-docstring/good.py

@@ -0,0 +1,6 @@
+"""Module providingFunction printing python version."""
+import sys
+
+
+def print_python_version():
+    print(sys.version)