documentation/python: format overloaded func ID only if it's overloaded.

documentation/python: format overloaded func ID only if it's overloaded.

So "pybound" functions without overloads still can have nice IDs and
URLs. This should have been done together with
1cd0bb44b77b4225fc5156e1ea7c1a790fe9f7a8 and
6db6b4c4f8296b0fa1b08ce6edeb942db321f6b4 already, but somehow I didn't
make the connection.