Not sure how long this has been hidden away here, but I just stumbled on a whole lot more specifier documentation than is included in Unreal Architecture section of the documentation. Looks like it lists pretty much everything, on the metadata side of things in particular. Figured others might find it useful.
Scroll down to near the bottom, in the Enums section. There are links labelled UP::, UC:: etc, and UM:: for all the different classes of metadata.