Initial module docstring for python _pb2 (#7528)

Generated Python modules now have a module-level docstring.

src/google/protobuf/compiler/python/python_generator.cc

@@ -183,7 +183,7 @@ void PrintTopBoilerplate(io::Printer* printer, const FileDescriptor* file,
       "# -*- coding: utf-8 -*-\n"
       "# Generated by the protocol buffer compiler.  DO NOT EDIT!\n"
       "# source: $filename$\n"
-      "\n",
+      "\"\"\"Generated protocol buffer code.\"\"\"\n",
       "filename", file->name());
   if (HasTopLevelEnums(file)) {
     printer->Print(