DOC: Clarify lack of index priority

[matthewfeickert]

• Specifically mention that --index-url and --extra-index-url both provide locations for pip to look for packages in the 'Finding Packages' section. However, explicitly state that there is no priority ordering for the search locations.
  • Text adapted from Why "pip install" does not respect my project.dependencies setting in the pyproject.toml? packaging-problems#736 (comment)
• This is a recurring point of confusion on the pip GitHub issue tracker as well as on https://github.com/pypa/packaging-problems/issues/, so additional clarification could help here.

Related Issues

• Why "pip install" does not respect my project.dependencies setting in the pyproject.toml? packaging-problems#736
• Clarify use of index-url vs. extra-index-url scientific-python/upload-nightly-action#76

[matthewfeickert]

Tagging for review/comments: @pfmoore (given they have given lots of suggestions on similar Issues in the past and they added this text originally in PR #3300), @henryiii (provided the original text which this PR adapts).

[matthewfeickert]

Thanks for the fast review @pfmoore!

[pfmoore]

Thanks for the contribution!