I'd say both documentations are very complete but take a different approach. Very often the python standard library is very self-descriptive and it's just doing what you would expect. If you're in doubt you can always start up bpython and have an autocompleted shell. That python has an interactive interpreter really is very convenient.
It's bit strange strange that you gave undocumented function as an example of how PHP functions are documented. :-)
This difference between Python docs and PHP docs is just the thing I noticed when I started having fun with Python which is btw awesome language though finding which lib to use for given task sometimes is hard.
Good thing to do to find out what python function does is running help(module_name.function_name) in interpreter.