bazel: Add comment for public protoc plugin

The 3rd-party rules_proto project is referencing our compiler directly
and not using our java_grpc_library. This target is fine for them to
use (although we'd prefer using our java_grpc_library), but most users
shouldn't be touching it.

Related to grpc#5942 and grpc#5947

compiler/BUILD.bazel

@@ -1,5 +1,6 @@
 load("//:java_grpc_library.bzl", "java_rpc_toolchain")
 
+# This should not generally be referenced. Users should use java_grpc_library
 cc_binary(
     name = "grpc_java_plugin",
     srcs = [