Add comments to mixed-cluster upgrade code

[janl]

Via #4394 (review)

It’d be nice if we tagged code that can be removed after we can expect everybody to have gone through a rolling cluster upgrade with the version number at which it can be removed, so we can semi-automate cleaning up said code.

We could decide to make it a promise that if someone wants to do a rolling upgrade from N-1 to N, that if they are on the latest N-1 release that they can upgrade safely and that N no longer has the upgrade code. But all versions inside N-1 can be rolling-upgraded.

That would reflect reality, especially outside IBM/Cloudant, where folks don’t track each and every CouchDB version.

cc @mikerhodes