sync::watch: Change wording from "unseen" to "changed"

[uklotzde]

Motivation

To keep the wording in the API consistent as discussed in #5962.

Solution

Rename the newly added (and yet unreleased) method mark_unseen() to mark_changed() in sync::watch::Receiver.

[uklotzde]

Ups, I got it wrong. The method must be called mark_changed(). Fixed

[uklotzde]

@victor-timofei Please check if these API changes are reasonable.

@Darksonn This PR could only be merged before 1.33 is released.

[uklotzde]

@victor-timofei I have deliberately decided to keep the existing wording of unseen at other places in the API docs. This synonym with a slightly different perspective helps to understand possible use cases.

[Darksonn]

This documentation is insufficient.

That said, this should probably have been done in the previous PR. Do you want to update the docs here? Otherwise I'll open an issue for it and merge this.

[uklotzde]

Not sure what is expected here. I have added some docs including a possible use case.

[Darksonn]

Thanks.