When using GNATdoc, or some similar too, documentation about the types and subprograms in a package are extracted from the Ada source of the package specification. It should be possible to include that documentation in the MXDR file and have Merc copy it into the generated files so that GNATdoc (or similar) can see it. It is natural to put the documentation on the different message types along with the declaration of the Message_Type enumeration, and to put the documentation on message parameters on the various encoding and decoding subprograms.
When using GNATdoc, or some similar too, documentation about the types and subprograms in a package are extracted from the Ada source of the package specification. It should be possible to include that documentation in the MXDR file and have Merc copy it into the generated files so that GNATdoc (or similar) can see it. It is natural to put the documentation on the different message types along with the declaration of the Message_Type enumeration, and to put the documentation on message parameters on the various encoding and decoding subprograms.