Document temporary .isorted files

[sapunyangkut]

Closes #1943

Summary

• explain when isort creates a sibling .isorted temporary file
• document normal and exception cleanup, plus possible residue after abrupt termination
• link to --overwrite-in-place as the no-temporary-file alternative and state its documented tradeoff

Validation

• built all documentation with the repository's strict Sphinx flags (--nitpicky --fail-on-warning --write-all --fresh-env)
• skipped only the theme's Google Fonts download in an untracked local validation config after fonts.google.com timed out
• verified the rendered section and internal --overwrite-in-place link
• checked default cleanup, exception cleanup, and overwrite-in-place behavior against the current implementation
• git diff --check

AI assistance

This documentation change was prepared with Codex assistance. I reviewed every statement against the current implementation, tightened the exception-cleanup wording after independent review, and ran the validation above.