Document the frameIndex parameter in the service protocol specification

runtime/vm/service/service.md

 # Dart VM Service Protocol 3.5
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
 This document describes of _version 3.5_ of the Dart VM Service Protocol. This
 protocol is used to communicate with a running Dart Virtual Machine.
 
 To use the Service Protocol, start the VM with the *--observe* flag.
 The VM will start a webserver which services protocol requests via WebSocket.
 It is possible to make HTTP (non-WebSocket) requests,
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 The _removeBreakpoint_ RPC is used to remove a breakpoint by its _id_.
 
 Note that breakpoints are added and removed on a per-isolate basis.
 
 See [Success](#success).
 
 ### resume
 
 ```
 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.
 
 If the _step_ parameter is not provided, the program will resume
 regular execution.
 
 If the _step_ parameter is provided, it indicates what form of
 single-stepping to use.
 
 step | meaning
 ---- | -------
 Into | Single step, entering function calls
 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
 
 ```
 Success setExceptionPauseMode(string isolateId,
                               ExceptionPauseMode mode)
 ```
 
 The _setExceptionPauseMode_ RPC is used to control if an isolate pauses when
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 2.0 | Describe protocol version 2.0.
 3.0 | Describe protocol version 3.0.  Added UnresolvedSourceLocation.  Added Sentinel return to getIsolate.  Add AddedBreakpointWithScriptUri.  Removed Isolate.entry. The type of VM.pid was changed from string to int.  Added VMUpdate events.  Add offset and count parameters to getObject() and offset and count fields to Instance. Added ServiceExtensionAdded event.
 3.1 | Add the getSourceReport RPC.  The getObject RPC now accepts offset and count for string objects.  String objects now contain length, offset, and count properties.
 3.2 | Isolate objects now include the runnable bit and many debugger related RPCs will return an error if executed on an isolate before it is runnable.
 3.3 | Pause event now indicates if the isolate is paused at an await, yield, or yield* suspension point via the 'atAsyncSuspension' field. Resume command now supports the step parameter 'OverAsyncSuspension'. A Breakpoint added synthetically by an 'OverAsyncSuspension' resume command identifies itself as such via the 'isSyntheticAsyncContinuation' field.
 3.4 | Add the superType and mixin fields to Class. Added new pause event 'None'.
 3.5 | Add the error field to SourceReportRange.  Clarify definition of token position.  Add "Isolate must be paused" error code.
 3.6 (unreleased) | Add 'scopeStartTokenPos', 'scopeEndTokenPos', and 'declarationTokenPos' to BoundVariable. Add 'PausePostRequest' event kind. Add 'Rewind' StepOption. Add error code 107 (isolate cannot resume). Add 'reloadSources' RPC and related error codes.
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss