doc: update go1.6.txt for cmd/cgo's C.complexfloat and C.complexdouble fix

Updates #13402.

Change-Id: Ia7b729d81fb78206d214444911f2e6573b88717a
Reviewed-on: https://go-review.googlesource.com/17240Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.6.txt

@@ -7,6 +7,7 @@ cmd/go: add -msan option (https://golang.org/cl/16169)
 cmd/go: use shallow clones for new git checkouts (https://golang.org/cl/16360)
 cmd/compile: add -msan option (https://golang.org/cl/16160)
 cmd/link: add -msan option (https://golang.org/cl/16161)
+cmd/cgo: fix C.complexfloat and C.complexdouble and treat as separate types from complex64 and complex128 (https://golang.org/cl/17208)
 
 Ports: