Let the Butler report be clearer on where to add Doxygen comments

[mfalkvidd]

The Butler report currently looks like this:

If keywords don't have Doxygen comments, they will not be available at https://www.mysensors.org/apidocs/index.html Add Doxygen comments to make it easier for users to find and understand how to use the new keywords.
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IT would help contributors if the Butler report was more clear on where to add the Doxygen comments. Maybe something like this:

If keywords don't have Doxygen comments, they will not be available at https://www.mysensors.org/apidocs/index.html Add Doxygen comments to MyConfig.h to make it easier for users to find and understand how to use the new keywords. You can use existing keywords in MyConfig.h as inspiration / template.
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