Document the frameIndex parameter in the service protocol specification

runtime/vm/service/service.md

Index: runtime/vm/service/service.md
diff --git a/runtime/vm/service/service.md b/runtime/vm/service/service.md
index d372c796fb03913afc17253be101adb5ac47df90..b9cdf8191be3ff7a815d5624917debb65e0a35b8 100644
--- a/runtime/vm/service/service.md
+++ b/runtime/vm/service/service.md
@@ -697,7 +697,8 @@ See [Success](#success).
 
 ```
 Success resume(string isolateId,
-               StepOption step [optional])
+               StepOption step [optional],
+               int frameIndex [optional])
 ```
 
 The _resume_ RPC is used to resume execution of a paused isolate.
@@ -715,6 +716,12 @@ Over | Single step, skipping over function calls
 Out | Single step until the current function exits
 Rewind | Immediately exit the top frame(s) without executing any code. Isolate will be paused at the call of the last exited function.
 
+The _frameIndex_ parameter is only used when the _step_ parameter is Rewind. It
+specifies the stack frame to rewind to. Stack frame 0 is the currently executing
+function, so _frameIndex_ must be at least 1.
+
+If the _frameIndex_ parameter is not provided, it defaults to 1.
+
 See [Success](#success), [StepOption](#StepOption).
 
 ### setExceptionPauseMode