Add a legend for the build prefixes

[nixjdm]

I don't think there's a simple key or legend for the various build prefixes visible anywhere. They're mostly self explanatory, but clarity always helps.

Namely, what do these all mean? (not that I don't know):

• U
• X (perhaps the least obvious)
• E
• D

[nixjdm]

Perhaps this could be outputted on a single stylized line after all build events are finished.