compiler/protogen: document --go_opt in addition to --go_out

Fixes golang/protobuf#1686

Change-Id: I88d6534ce5159dc48053949fe42e6f9eeb7d5bdf
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/737460
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>

compiler/protogen/protogen.go

diff --git a/compiler/protogen/protogen.go b/compiler/protogen/protogen.go
index 4af705f..6db0385 100644
--- a/compiler/protogen/protogen.go
+++ b/compiler/protogen/protogen.go
@@ -131,10 +131,11 @@
 	// generator parameter.
 	//
 	// Plugins for protoc can accept parameters from the command line,
-	// passed in the --<lang>_out protoc, separated from the output
-	// directory with a colon; e.g.,
+	// passed in the --<lang>_opt protoc flag, in addition to the
+	// (required) output directory in --<lang>_out; e.g.,
 	//
-	//   --go_out=<param1>=<value1>,<param2>=<value2>:<output_directory>
+	//   --go_opt=<param1>=<value1>,<param2>=<value2>
+	//   --go_out=<output_directory>
 	//
 	// Parameters passed in this fashion as a comma-separated list of
 	// key=value pairs will be passed to the ParamFunc.

internal/cmd/generate-protos/main.go

diff --git a/internal/cmd/generate-protos/main.go b/internal/cmd/generate-protos/main.go
index 7eb6a9f..305f351 100644
--- a/internal/cmd/generate-protos/main.go
+++ b/internal/cmd/generate-protos/main.go
@@ -411,7 +411,7 @@
 					opts += fmt.Sprintf(",apilevelM%v=%v", relPath, "API_OPEN")
 				}
 			}
-			protoc("-I"+filepath.Join(repoRoot, "src"), "-I"+filepath.Join(protoRoot, "src"), "-I"+repoRoot, "--go_out="+opts+":"+tmpDir, filepath.Join(repoRoot, relPath))
+			protoc("-I"+filepath.Join(repoRoot, "src"), "-I"+filepath.Join(protoRoot, "src"), "-I"+repoRoot, "--go_opt="+opts, "--go_out="+tmpDir, filepath.Join(repoRoot, relPath))
 			return nil
 		})