go/token: document postcondition of SetLines

Change-Id: Ie163deade396b3e298a93845b9ca4d52333ea82a
Reviewed-on: https://go-review.googlesource.com/22831Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/token/position.go

@@ -164,6 +164,7 @@ func (f *File) MergeLine(line int) {
 // Each line offset must be larger than the offset for the previous line
 // and smaller than the file size; otherwise SetLines fails and returns
 // false.
+// Callers must not mutate the provided slice after SetLines returns.
 //
 func (f *File) SetLines(lines []int) bool {
 	// verify validity of lines table