compiler/protogen: improve documentation

Change-Id: I68e918d297716178bc1d9f459edb1cd1678d4159
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220340
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/compiler/protogen/protogen.go b/compiler/protogen/protogen.go
index 8ffccca..6d21313 100644
--- a/compiler/protogen/protogen.go
+++ b/compiler/protogen/protogen.go
@@ -4,10 +4,10 @@
 
 // Package protogen provides support for writing protoc plugins.
 //
-// Plugins for protoc, the Protocol Buffers Compiler, are programs which read
-// a CodeGeneratorRequest protocol buffer from standard input and write a
-// CodeGeneratorResponse protocol buffer to standard output. This package
-// provides support for writing plugins which generate Go code.
+// Plugins for protoc, the Protocol Buffer compiler,
+// are programs which read a CodeGeneratorRequest message from standard input
+// and write a CodeGeneratorResponse message to standard output.
+// This package provides support for writing plugins which generate Go code.
 package protogen
 
 import (