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

[CoretexShadow]

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