Simplify preview warning

[richlander]

This multi-line warning bugs me. I'd like it to be single line.

• Not sure if the localized files need to be updated. I didn't do that, but can.
• The link (same as old one) isn't that helpful. We should have a doc on using previews. Global.json is one part of using previews.

[livarcocc]

cc @KathleenDollard to review.

I will take it to shiproom when @KathleenDollard reviews it.

[KathleenDollard]

Let's fix this according to Rich's suggestion.

[livarcocc]

Actually, at this point, I think we should take this for 2.1.4xx. @richlander , I will take your commit, put it in my own branch, fix the test failures and re-target it to 2.1.4xx.

[wli3]

seems miss xlf update. You need to run ./build.cmd -p:UpdateXlfOnBuild=true and check in the result

[nguerrera]

Replaced by #2286