Summary

Adds documentation clarifying that urllib.parse.urlsplit() and
urllib.parse.urlparse() should not be relied on solely for validating
redirect targets using the netloc component.

Certain URL forms may produce an empty netloc value while being interpreted
differently by user agents such as web browsers. This can lead to incorrect
assumptions when applications use netloc checks to prevent open redirects.

Changes

• Added security documentation explaining the limitation of netloc checks.
• Included an example showing how urlsplit() handles URLs with multiple
  leading slashes.
• Clarified that applications handling redirects should perform validation
  appropriate for their security requirements.

Issue

Fixes #142412

Notes

This is a follow-up to #144448, incorporating the review feedback by placing
the guidance within the existing "URL parsing security" section rather than
adding a separate warning block.

Testing

• Documentation change only.
• Verified the documentation formatting locally.

• Issue: Warn about urlsplit()'s netloc not being suitable for open redirect checking #142412