Explicitly mention use of seed packages during `uv venv --seed` (#4588)
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Closes #1329.
## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
Mentions use of seed packages during `uv venv --seed`, and clarifies the
divergence in behavior when using Python 3.12+.
## Test Plan
<!-- How was it tested? -->
`cargo nextest run --test venv`
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Co-authored-by: Zanie Blue <contact@zanie.dev>