cmd/go: document usage of external test packages

Fixes #5144

R=golang-dev, r, dave, adg
CC=golang-dev
https://golang.org/cl/8171044

src/cmd/go/doc.go

@@ -398,6 +398,9 @@ the file pattern "*_test.go".  These additional files can contain test functions
 benchmark functions, and example functions.  See 'go help testfunc' for more.
 Each listed package causes the execution of a separate test binary.
 
+Additionally, test files that declare a package with the suffix "_test" will be
+compiled and executed separately from the package test binary.
+
 By default, go test needs no arguments.  It compiles and tests the package
 with source in the current directory, including tests, and runs the tests.
 

src/cmd/go/test.go

@@ -50,6 +50,9 @@ the file pattern "*_test.go".  These additional files can contain test functions
 benchmark functions, and example functions.  See 'go help testfunc' for more.
 Each listed package causes the execution of a separate test binary.
 
+Additionally, test files that declare a package with the suffix "_test" will be
+compiled and executed separately from the package test binary.
+
 By default, go test needs no arguments.  It compiles and tests the package
 with source in the current directory, including tests, and runs the tests.