Add descriptions to label attributes exposed by LabelMeta

[LandingEllipse]

While we'll eventually have dedicated documentation for EXM and PanCam label attributes a la NASA's HTML approach, it would arguably be valuable for application programmers if the attributes exposed via LabelMeta had some level of inline documentation. Partly to disambiguate the moniker to label path mappings, but also to lower the need for programmers to switch context (i.e., to the external docs) during development.

Ideally, the implementation would pull from a central source to avoid the maintenance burden of fragmented/duplicated documentation. Not immediately obvious how that would be achieved while remaining IM/DD/LDD version agnostic, though it could be argued that Proctools is implicitly tied anyway.