doc/go1.5.txt: add note about internal

Change-Id: I6476284a2cf80d50bd0a57fd9a0de9bc74273c7e
Reviewed-on: https://go-review.googlesource.com/10680Reviewed-by: Russ Cox <rsc@golang.org>

doc/go1.5.txt

@@ -3,6 +3,7 @@ Overall:
 - new GC
 - go tool asm, go tool compile, go tool link
 - default output files changed: now file.o and a.out
+- internal enforced even outside standard library (golang.org/s/go14internal; https://golang.org/cl/9156)
 
 Language:
 - permit omission of key type in map composite literals where key is a composite literal (https://golang.org/cl/2591)