Clarify definition of token position.

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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 }
 ```
 
 A _Script_ provides information about a Dart language script.
 
 The _tokenPosTable_ is an array of int arrays. Each subarray
 consists of a line number followed by _(tokenPos, columnNumber)_ pairs:
 
 > [lineNumber, (tokenPos, columnNumber)*]
 
+The _tokenPos_ is an arbitrary integer value that is used to represent
+a location in the source code.  A _tokenPos_ value is not meaningful
+in itself and code should not rely on the exact values returned.
+
 For example, a _tokenPosTable_ with the value...
 
 > [[1, 100, 5, 101, 8],[2, 102, 7]]
 
 ...encodes the mapping:
 
 tokenPos | line | column
 -------- | ---- | ------
 100 | 1 | 5
 101 | 1 | 8
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 version | comments
 ------- | --------
 1.0 | initial revision
 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.
+3.5 | Add the error field to SourceReportRange.  Clarify definition of token position.
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss