Add suggested polling frequency (aka "shelf life") to decision point pages

[ahouseholder]

Describe the solution you'd like

(based on a suggestion from @laurie-tyz)

/docs/howto/bootstrap/use.md has a section Information Changes Over Time.

In it are some suggestions for individual decision points and "polling frequency". We should include this information on the individual decision point pages as well, where appropriate.

Describe alternatives you've considered

• Status quo: keep the content on the use.md page only. Folks who only see the decision point pages in /docs/reference/decision_points might never encounter it.

Additional context

"Shelf life" may or may not be relevant to every decision point, but we should consider whether it is or not in the course of resolving this issue.

This is parallel in concept to the subject of #522, which also suggests replicating information from the bootstrapping docs into individual decision point reference pages.

[j---]

This seems like a good idea.
Capturing some notes for future us, before I forget:
Some decision points will be on the order of 1-2 years, for example Mission Impact might be re-validated rather rarely. I suppose CVSS Base score policy is that CVSS vectors are static unless challenged. So I could see an argument that Technical Impact or Value Density are never re-checked. If that is true, it would make sense to document that, also.