Add comment about futures

log.go

@@ -46,6 +46,9 @@ type ParseCPFunc func(raw []byte) (*f_log.Checkpoint, error)
 type EntriesPathFunc func(n, logSize uint64) string
 
 // IndexFuture is the signature of a function which can return an assigned index or error.
+//
+// Implementations of this func are likely to be "futures", or a promise to return this data at
+// some point in the future, and as such will block when called if the data isn't yet available.
 type IndexFuture func() (uint64, error)
 
 // StorageOptions holds optional settings for all storage implementations.