cmd/go: indicate that flags unrecognized by 'go test' must follow package specification

R=rsc, minux.ma
CC=golang-dev
https://golang.org/cl/7300063

src/cmd/go/doc.go

@@ -766,6 +766,8 @@ will compile the test binary and then run it as
 The test flags that generate profiles also leave the test binary in pkg.test
 for use when analyzing the profiles.
 
+Flags not recognized by 'go test' must be placed after any specified packages.
+
 
 Description of testing functions
 

src/cmd/go/test.go

@@ -167,6 +167,8 @@ will compile the test binary and then run it as
 
 The test flags that generate profiles also leave the test binary in pkg.test
 for use when analyzing the profiles.
+
+Flags not recognized by 'go test' must be placed after any specified packages.
 `,
 }