docs: Mention env var PYTHONUNBUFFERED

[quassy]

Summary

I did not find any info about whether uv (run in particular) supports PYTHONUNBUFFERED but it seems to be implemented in env_vars.rs.

(Sadly still no CLI parameter -u exists.)

Test Plan

No testing as is purely a change in the markdown documentation.

[charliermarsh]

Thanks for raising this! I think we tend to avoid documenting environment variables that affect python, but that we don't read or write directly (which includes PYTHONUNBUFFERED). If we add PYTHONUNBUFFERED, then we should probably add every environment variable that impacts the python runtime, which seems sort of untenable.

(We do have PYTHONUNBUFFERED in env_vars.rs, but it's only used when we compile bytecode, where we set it to "1" for a python process that we run within uv. That's why it's not exposed.)