The documentation for `MultipartConversionError` is outdated

Documentation

The document mentions the add_payload method, but add_payload no longer exists and has been replaced by attach. This exception seems to only be raised by attach.

Linked PRs

• gh-130433: Update documentation for MultipartConversionError #130436
• [3.13] gh-130433: Update documentation for MultipartConversionError (GH-130436) #130565
• [3.12] gh-130433: Update documentation for MultipartConversionError (GH-130436) #130566

[encukou]

Thank you!