Target: Guidelines, primer

Justification: 

MTOM is a very bad example for optionality since it raises ambiguities depending on message flow. Best practices might suggest using different endpoints for with and without MTOM. Reason is that MTOM assertion mandates usage of MTOM on both requests and responses, so request without MTOM is ambiguous, is it a message that wouldn't naturally use MTOM, is it in error, or does it signal desire not to use MTOM.

Proposal:
Change to example using WS-SecurityPolicy, for example that messages optionally may need timestamp using Timestamp assertion.

Test: review guidelines and primer.