gh-142474: Clarify -i interaction with PYTHONSTARTUP

[Joshua-Ward1]

Summary

Clarify the documentation for the -i command-line option so that it accurately reflects how it interacts with PYTHONSTARTUP.

The updated text distinguishes the two distinct behaviors of -i:

• When -i is used with a script or with -c, Python executes the script or command and then enters interactive mode. In these “execute then interact” cases, PYTHONSTARTUP is not read.

• When -i is used solely to force interactive mode even if stdin is not a terminal (for example with redirected standard input), the interpreter enters interactive mode directly and PYTHONSTARTUP is read as usual. The previous documentation implied it was never read when using -i.

Issue number

Closes gh-142474.

Type of change

• [x] Documentation change only.

Testing

• [x] Built documentation locally (make html) to confirm RST formatting.
• [x] Verified that the updated wording matches the current behavior of -i and PYTHONSTARTUP.

Notes

• Per .github/CONTRIBUTING.rst, no NEWS entry is required for this small documentation clarification.

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📚 Documentation preview 📚: https://cpython-previews--142502.org.readthedocs.build/

[python-cla-bot[bot]]

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[FFY00]

@vstinner, would you mind giving this a look?