Added missing-function-docstring message example (#6248)

Author: matusvalo

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

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

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

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