Address review comments.

Author: coryan

CMakeLists.txt

@@ -137,6 +137,10 @@ function (googleapis_cpp_short_name var proto)
     set("${var}" "${short_name}" PARENT_SCOPE)
 endfunction ()
 
+# Create a single source proto library.
+#
+# - proto: the filename for the proto source.
+# - (optional) ARGN: proto libraries the new library depends on.
 function (googleapis_cpp_add_library proto)
     googleapis_cpp_short_name(short_name "${proto}")
     google_cloud_cpp_grpcpp_library(googleapis_cpp_${short_name}