Deprecation warning about in-place builds is confusing/ambiguous

[maresb]

Description

I have trouble understanding the text of the deprecation message:

DEPRECATION: A future pip version will change local packages to be built in-place without first copying to a temporary directory. We recommend you use --use-feature=in-tree-build to test your packages with this new behavior before it becomes the default.
pip 21.3 will remove support for this functionality. You can find discussion regarding this at #7555.

Expected behavior

I would expect that after reading the deprecation message once, I understand roughly what is going to be changed, what will be removed in pip 21.3, and what I can do about it.

For more details, see #7555 (comment)

pip version

21.1.3

Python version

3.9.5

OS

Linux

How to Reproduce

Run pip install .

Output

DEPRECATION: A future pip version will change local packages to be built in-place without first copying to a temporary directory. We recommend you use --use-feature=in-tree-build to test your packages with this new behavior before it becomes the default.
   pip 21.3 will remove support for this functionality. You can find discussion regarding this at https://github.com/pypa/pip/issues/7555.

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[pradyunsg]

Whee! Thanks for filing this issue and PR separately @maresb! Much appreciated! ^>^