Module doc error when deriving protocols on modules you don't own

[tristantreb]

Context: I implemented protocols to encode to JSON as advised here,
"""
If you don't own the struct you want to encode to JSON, you may use Protocol.derive/3 placed outside of any module:

Protocol.derive(Jason.Encoder, NameOfTheStruct, only: [...])
Protocol.derive(Jason.Encoder, NameOfTheStruct)

"""

Issue: mix doctor complains that there's no module doc when I derive a Protocol from a module I don't own.

Is it a bug?
The best workaround I found is to ignore the related modules from .doctor.exs