Obviously the prototype can't be provided when it isn't known.  But the pseudo-English text is trying to describe the prototype and is far less clear than the actual prototype.  ie the developer communicated the prototype to Python in Python syntax, why not have Python do the same rather than use a different and very unfamiliar language.  (Try using the Python documentation to work out what the word 'positional' means.)

As for looking it up in the source code, that is one of the harder things for newbies to do, and certainly very difficult for a lot of people.  How would they know where the standard library source is on their system?  Heck I don't even know where the source is for third party packages on my Ubuntu system as the last I looked they used convoluted sequences of version numbers, symbolic links and who knows what else.  The popularity of eggs makes it even harder to look up.