docs(pip_freeze): clarify --all; add concise example

[CortexShadow]

Summary
Improves the documentation for pip freeze --all by adding a clear explanatory note and a before/after example. This is a documentation-only change; no behavior is modified.

Motivation
The current description of --all can be misread and leaves ambiguity about which packages are shown. By making it explicit that pip, setuptools, and wheel are skipped by default—and included with --all—the docs now provide a more accurate and practical mental model for users.

Changes

• Added a .. note:: clarifying default behavior and the effect of --all.
• Introduced Unix and Windows examples comparing pip freeze vs. pip freeze --all.
• Kept the style consistent with the CLI reference page.

Notes

• Local linters, pre-commit hooks, targeted freeze tests, and the Sphinx docs build all pass.
• No changelog/news entry is required since this is docs-only.

Related
Closes #13557

[notatallshaw]

Hi @CoretexShadow thanks for submitting this PR to pip!

All pip maintainers currently operate on a volunteer basis, so it can take some time for one of to take a look and review a PR. I will try and make some time soon to review this PR though.

[sepehr-rs]

Hi @CortexShadow, thanks a lot for contributing to pip!
I’m not on the triage team yet, so I can’t formally approve, but I checked the updated docs and they’re consistent with pip’s behavior. This looks good to me!

[CortexShadow]

Hi @CortexShadow, thanks a lot for contributing to pip! I’m not on the triage team yet, so I can’t formally approve, but I checked the updated docs and they’re consistent with pip’s behavior. This looks good to me!

Thanks for checking this and confirming! Really appreciate your feedback.