[12.x] Add missing @param docblock to ValidatedInput::__isset()

[ismaildasci]

This PR adds the missing @param string $name docblock annotation to the __isset() magic method in ValidatedInput.

Consistency: The other magic methods in this class (__get, __set, __unset) all have their @param annotations properly documented, but __isset() was missing it.

Before:

/**
 * Determine if an input item is set.
 *
 * @return bool
 */
public function __isset($name)

After:

/**
 * Determine if an input item is set.
 *
 * @param  string  $name
 * @return bool
 */
public function __isset($name)