There's a description property used for specifying the optional description of a commit. But this property can surely be used in many other cases, to provide human- or machine-written textual descriptions of objects. The current standard options are:
If there's an existing description property in some ontology such as DOAP or DC etc., we could reuse it here (if the thing with mediaType works out; it would likely mean we drop mediaType and just always treat it as plain text, which is hopefully fine). IIRC I added mediaType there because the GAnarchy person mentioned treating the description as Markdown, so there's a use case for varying media type.