Improve docs around setup_requires

[gsnedders]

Description

Currently, pip has a bunch of docs about setuptools's old setup_requires: https://pip.pypa.io/en/stable/reference/build-system/#controlling-setup-requires

These have a number of problems:

• The end-user section could be more clearly split between easy_install and pip sections, and make the mention that easy_install is used by setuptools < 52.0 in the main body of the text.
• The end-user section talks about how to ensure easy_install invocations don't reach PyPI, but it doesn't give any comparable advice or commentary about more recent setuptools versions that use pip. It says in the historical context box that there's "limited ability to pass through any command line arguments", so does that mean you just can't stop it from hitting a different index to the user-initiated pip or is that passed through by default? At the very least, the behaviour should be clarified.
• It doesn't cover that --use-pep517 does make the current pip install the requirements, given they're passed back from setuptools per the normal build interface. This seems like it may actually be a better solution than "the best solution".

Expected behavior

No response

pip version

N/A

Python version

N/A

OS

N/A

How to Reproduce

N/A

Output

No response

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[notatallshaw]

I assume any improvements in doc clarity would be welcome.

[zahlman]

Shouldn't it be Setuptools' responsibility now to explain the easy_install configuration, not Pip's? (I think the current state of the documentation is the result of Pip and Setuptools' history of being intertwined.) If they already have something, perhaps that should just be linked from this part of the doc.