Temporal safety information is outdated

[rwatson]

When Section 3.2 (Referential, spatial, and temporal safety), which contains various memory-safety definitions, was written in 2020, we hadn't yet deployed heap temporal safety in CheriBSD. This text requires updating.

Likewise Section 4.6 (Implications of capability revocation for temporal safety), which talks about its implications, needs updating.

There may be other places where documentation could be updated with respect to temporal safety, and perhaps also discussion of API impacts.