The note about the signature of the ReflectionParameter constructor is actually incomplete, at least in 5.2.5: it is possible to use an integer for the second parameter, and the constructor will use it to return the n-th parameter.
This allows you to obtain proper ReflectionParameter objects even when documenting code from extensions which (strangely enough) define several parameters with the same name. The string-based constructor always returns the first parameter with the matching name, whereas the integer-based constructor correctly returns the n-th parameter.
So, in short, this works:
<?php
$p = new ReflectionParameter(array('Some_Class', 'someMethod'), 4);
$p = new ReflectionParameter(array('Some_Class', 'someMethod'), 'y');
?>