add deprecation

Author: alexeagle

python/proto/BUILD.bazel

@@ -15,6 +15,8 @@
 package(default_visibility = ["//visibility:public"])
 
 # Toolchain type provided by proto_lang_toolchain rule and used by py_proto_library
+# Deprecated in favour of the implementation in https://github.com/protocolbuffers/protobuf.
+# It will be removed in a future release.
 # Alias is here to provide backward-compatibility; see #2604
 alias(
     name = "toolchain_type",