doc: revise deprecation semverness info in Collaborator Guide

Simplify and clarify deprecation semverness information in the Collaborator Guide. Unlike some of the other changes I've made lately, this one is not merely cosmetic. It changes information about how to handle deprecations vis-a-vis SemVer. The revised conventions take advange of notable change labels etc. instead of suggesting that doc-deprecations be treated as semver-minor. The idea that a deprecation is a new feature seems incorrect from a SemVer perspective, but probably made sense at the time the text was written if we weren't yet using notable change etc.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines