Skip implicit parameters if explicit ones exist.

For example with code like this:

    /**
     * My Function
     * @param {Number} foo
     */
    function blah(foo, bar) {
    }

The previous behaviour of JSDuck was to include both parameters
to documentation.  After this change only the first parameter
will be documented, allowing us to easily have hidden parameters
for internal use, which is quite common in ExtJS.