Add canned explanation about feature preview flag to deprecated().

[maresb]

Closes #10165

[maresb]

Thanks @uranusjr so much for the review! I really appreciate it. I'm on the road at the moment but I will try to address everything soon.

[maresb]

Hello @uranusjr, I have attempted to address all of the points you raised. I am ready for the next round of review. I'm happy to further elaborate on my motivation, or to rebase/squash my commits. Thank you!

[pradyunsg]

I am a -1 on this PR as it stands -- more context at #10165 (comment).

[pypa-bot]

Hello!

I am an automated bot and I have noticed that this pull request is not currently able to be merged. If you are able to either merge the master branch into this pull request or rebase this pull request against master then it will eligible for code review and hopefully merging!

[maresb]

Closes #10223

[maresb]

@pradyunsg and @uranusjr, do the latest commits satisfactorily address all your concerns?

[uranusjr]

One minor thought, generally looks good enough to me.

[maresb]

Oops, I need to adjust the unit tests now...

[maresb]

@uranusjr and @pradyunsg, unit tests are passing, and from my side things look ready to go. Thus I squashed my commits to tidy things up. Any final comments?

[uranusjr]

The repeated formatted_xxx = XXX.format() if foo else None feels cumbersome, but it does the job, and functionality-wise lgtm.

[pradyunsg]

Thanks @maresb! I quite like where we ended up here, functionality wise! ^>^

[maresb]

Great, thanks so much for the merge @pradyunsg! I agree that this ended well, I like the current solution much better than my original proposal, so thank you and @uranusjr for the collaboration.