[docs] add explain about where to set use_builtin_types when use customTransport

[peng yu]

I'm using xmlrpc, and I'm using a custom Transport, follow the
"21.26.8. Example of Client Usage" section of below document:
https://docs.python.org/3/library/xmlrpc.client.html
I found if I use a custom transport, set use_builtin_types parameter
to xmlrpc.client.ServerProxy will have no effect. After read the
source code, I found if I use a custom transport, I should set the
use_builtin_types parameter in the transport. I think it is better to
describe this behavior in the document.

best regards.