email: documenting different behavior of trailing newline in `set_content` and `set_payload`

[ZeroIntensity]

Documentation

See #121515, which reported that set_content() implicitly adds a newline to the end of the content (if it doesn't have one already), whereas set_payload() ignores it. Unfortunately, this would be a breaking change to fix, so we should just document this behavior.

Linked PRs

• gh-121542: Document trailing newline behavior in set_content() #121543
• [3.13] gh-121542: Document trailing newline behavior in set_content() (GH-121543) #128995
• [3.12] gh-121542: Document trailing newline behavior in set_content() (GH-121543) #128996