[release-branch.go1.5] cmd/go: -a does apply to the standard library

This changed in https://golang.org/cl/10761.

Update #12203.

Change-Id: Ia37ebb7ecba689ad3cb2559213d675f21cf03a95
Reviewed-on: https://go-review.googlesource.com/13799Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/14226Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/go/alldocs.go

@@ -81,7 +81,6 @@ and test commands:
 
 	-a
 		force rebuilding of packages that are already up-to-date.
-		In Go releases, does not apply to the standard library.
 	-n
 		print the commands but do not run them.
 	-p n

src/cmd/go/build.go

@@ -60,7 +60,6 @@ and test commands:
 
 	-a
 		force rebuilding of packages that are already up-to-date.
-		In Go releases, does not apply to the standard library.
 	-n
 		print the commands but do not run them.
 	-p n