Within the functions section of the manual.
Most newcomers don't. That's why it's such a trap. The bigger picture is that eventually you will be working with someone else's code. Now, lets say they have a function documented like so....
This says that the function foo excepts 1 argument, a string to be used within the function as $name. Now, if this function also relies upon a global $content you have no way of knowing about it and you cannot use the function without first defining $content somewhere.
Functions accept arguments so that they are self documenting, sure, you could go and look at the code for the foo() function but you shouldn't have to.