PEP 694: Several updates based on Michał Górny's feedback on DPO

[warsaw]

https://discuss.python.org/t/pep-694-pypi-upload-api-2-0/76316/2

Summary:

• Add some error return code descriptions where they were underspecified
• Fix some misspellings and poorly worded text
• Combine the canceling and deleting of upload files sections
• Simplify the rules for replacing a staged but not yet published file
• Add open question about deferring stage previews
• Update Post-History header

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📚 Documentation preview 📚: https://pep-previews--4323.org.readthedocs.build/