Settle on definition of "pending revision" [#2940904]

Problem/Motivation

@plach in #2860097-129: Ensure that content translations can be moderated independently:

Open another follow-up to finally discuss the definition of pending revision, how different behaviors can be reconciled or at least make sure they don't conflict with each other, and possibly re-enable latest revision in any entity form. Basically continue on the foundations laid by #2860097-126: Ensure that content translations can be moderated independently, which looks like a promising start. Since we were ready to perform this change in between 8.4 and 8.5, it should be ok to perform it in any minor, but I'm not sure we will be able to make that happen in 8.5.

@Sam152 in #2860097-126: Ensure that content translations can be moderated independently:

My question and the source of disagreement is: what are the expectations developers have with regards to how core entity forms behave when a pending revision is created? In the content_moderation use case, ignoring those pending revisions in entity forms is data loss. Are other (current or future) consumers of this api in the same boat or are there concrete use cases out there where this would mean data leaking instead of data being preserved? The entry point into the whole API doesn't strictly define exactly what "pending" or "forward" implies exactly.

Is it helpful to be more or less opinionated about this? Are we preventing headaches for someone with $usecase or making false assumptions which are equally tricky to undo. I think I'm biased towards the former simply based on content_moderation being the "flagship" consumer of this API. I think "latest" and "pending" have some implication towards "this is the point at which things progress" more than "this is a generalised place to store more information". Purely theoretical, but if we nudged integrators towards this paradigm, is there be more scope for things to play nicely together?

The flip side of all of this breaking BC really sucks. My belief is still that this should be treated as fixing a critical bug, furthering the efforts to make the latest revision a first class citizen however @hchonov makes great points that this should be carefully evaluated.

So:

the concept of "pending revision" is open to interpretation, a clear documented definition would help avoid repeating the same discussion on many issues
define how different kinds of forward revisions ("pending revisions" are just one kind) can co-exist, for example #2860097-128: Ensure that content translations can be moderated independently (Actually […])