to_additive newline docstrings

Stream: mathlib4

[Moritz Doll]

Mathport creates newline characters for each new line in a custom to_additive docstring, this looks like a bug to me. See for instance https://github.com/leanprover-community/mathlib4/pull/2245/commits/b3ae0cac7907c7eb93be92fd597728e5c09856d0#diff-1d107bcd0d4cca7c26f9a7005856533de8d351272105bf7ad5646001eec5ee06R663