[Diagnostics] Update educational note for AsyncCallerExecution feature to match @execution attribute split

Author: xedin

userdocs/diagnostics/async-caller-execution.md

@@ -3,11 +3,14 @@
 Proposed in [SE-0461], this feature changes the behavior of nonisolated async
 functions to run on the actor to which the caller is isolated (if any) by 
 default, and provides an explicit way to express the execution semantics for
-these functions:
-* The `@execution(concurrent)` attribute specifies that a function must always 
+these functions.
+
+This feature was proposed in [SE-0461](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0461-async-function-isolation.md)
+
+* The `@concurrent` attribute specifies that a function must always 
   switch off of an actor to run.
   This is the default behavior without `AsyncCallerExecution`.
-* The `@execution(caller)` attribute specifies that a function must always 
+* The `nonisolated(nonsending)` modifier specifies that a function must always 
   run on the caller's actor.
   This is the default behavior with `AsyncCallerExecution`.