[dev.cc] doc/go1.5: mention cmd/go meaning of std change

Change-Id: I259e88f019b6818c57caaa1ec236b7c2e2ae1382
Reviewed-on: https://go-review.googlesource.com/5551Reviewed-by: Rob Pike <r@golang.org>

doc/go1.5.txt

@@ -23,6 +23,7 @@ testing/quick: support generation of arrays (https://golang.org/cl/3865)
 
 Tools:
 
+cmd/go: std wildcard now excludes commands in main repo (https://golang.org/cl/5550)
 cmd/vet: better validation of struct tags (https://golang.org/cl/2685)
 cmd/ld: no longer record build timestamp in Windows PE file header (https://golang.org/cl/3740)