doc/go1.6.txt: mention possible GOTRACEBACK change

For CL 16512, #12366, #13107.

Change-Id: I0ed1bb9597ac3db3fa35a037c304060d5a7e6d51
Reviewed-on: https://go-review.googlesource.com/16513Reviewed-by: Austin Clements <austin@google.com>

doc/go1.6.txt

@@ -20,6 +20,7 @@ image: add NYCbCrA types (https://golang.org/cl/15671)
 math/rand: add Read (https://golang.org/cl/14522)
 net/http: HTTP/2.0 support (many CLs)
 net/url: make *url.Error implement net.Error (https://golang.org/cl/15672)
+runtime: only one goroutine in traceback (https://golang.org/cl/16512) maybe
 strconv: QuoteToGraphic (https://golang.org/cl/14184/)
 text/template: ExecError (https://golang.org/cl/13957/)
 text/template: trimming spaces (https://golang.org/cl/14391/)