cmd/go: document test -json

Added in CL 76873 but forgot to add the flag docs.

Change-Id: I448f85cc8cf51399c0d799691226b94f88f327da
Reviewed-on: https://go-review.googlesource.com/77030
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Caleb Spare <cespare@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/alldocs.go

@@ -813,6 +813,10 @@
 // 	    Install packages that are dependencies of the test.
 // 	    Do not run the test.
 //
+// 	-json
+// 	    Convert test output to JSON suitable for automated processing.
+// 	    See 'go doc test2json' for the encoding details.
+//
 // 	-o file
 // 	    Compile the test binary to the named file.
 // 	    The test still runs (unless -c or -i is specified).

src/cmd/go/internal/test/test.go

@@ -143,6 +143,10 @@ In addition to the build flags, the flags handled by 'go test' itself are:
 	    Install packages that are dependencies of the test.
 	    Do not run the test.
 
+	-json
+	    Convert test output to JSON suitable for automated processing.
+	    See 'go doc test2json' for the encoding details.
+
 	-o file
 	    Compile the test binary to the named file.
 	    The test still runs (unless -c or -i is specified).