clarify that the defualt aggregation enables OptIn instruments

Author: dashpole

specification/metrics/sdk.md

@@ -1011,7 +1011,10 @@ This advisory parameter applies to all aggregations.
 
 When an instrument is `OptIn`, the SDK MUST use the
 [Drop Aggregation](#drop-aggregation) by default. If the user has provided an
-Aggregation via View(s), that aggregation takes precedence.
+Aggregation via View(s), that aggregation takes precedence. If the user
+provides the [Default Aggregation](#default-aggregation) using a View, this
+"enables" the instrument with the same behavior as-if the instrument was not
+`OptIn`--including respecting other advisory parameters.
 
 ### Instrument enabled