Clarify async instrument callback identity

specification/metrics/api.md

@@ -355,9 +355,10 @@ Where the API supports registration of `callback` functions after
 asynchronous instrumentation creation, the user MUST be able to undo
 registration of the specific callback after its registration by some means.
 
-Every currently registered Callback associated with an instrument MUST
-be evaluted exactly once during collection prior to reading data for
-that instrument.
+Every registered Callback associated with a set of instruments MUST
+be evaluated exactly once during collection prior to reading data for
+that instrument. The evaluated is identified using a combination of the callback
+itself, and the instruments set.
 
 Callback functions MUST be documented as follows for the end user: