added informative comment

bs/TagEngine.bs

@@ -11,6 +11,10 @@ interface (TagEngine :: # -> *) numTags =
     requestTag :: ActionValue UIntLog2N(numTags)
     retireTag  :: UIntLog2N(numTags) -> Action
 
+-- The tag engine returns a tag that is unique for the duration of
+-- the lifetime of the tag. Useful when you need to tag transactions
+-- on a bus for example.
+-- This implementation is stack based.
 mkTagEngine :: Module (TagEngine numTags)
 mkTagEngine =
     do