Advisory Notes - time limits

[SteveLLamb]

https://doc.smpte-doc.org/standards-operations-manual/main/#Advisory-Note

Discussion with @raymondyyeungDBL - Update to reflect that AN should be withdrawn when appropriate, not auto after 6 months.

For example:

• Withdrawn notice, should stay active permanently
• Bugs, should stay active until issue is resolved and fixed
• Other use cases might be applicable

Comments to be added for specific wording on revision from @raymondyyeungDBL.

[SteveLLamb]

@fschleich - @raymondyyeungDBL and I think this should be on the list for next revision.