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.

[DanielNoord]

Is this ready to be reviewed?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.39%. Comparing base (a8c0004) to head (3b5ac12).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2580   +/-   ##
=======================================
  Coverage   99.39%   99.39%           
=======================================
  Files          41       41           
  Lines        3137     3137           
  Branches      680      680           
=======================================
  Hits         3118     3118           
  Misses         11       11           
  Partials        8        8           

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