Replace Nullable ? in docblocks with |null verbosely.

[dereuromark]

Resolves #16

But it seems that union sniffs with more than a single type added dont get fixed up yet..

     protected $string1 = null;
 
     /**
-     * @var string|int|null
+     * @var ?string|int
      */
     protected $string2 = null;
 
     /**
      * @param string|null $string1
-     * @param string|null $string2
+     * @param ?string|null $string2
      *
      * @return string|null Some Comment
      */

Rest seems to work

[dereuromark]

If I var_dump all the snippets process() runs into, those are

string(20) "?string Some Comment"
string(11) "?string|int"
string(16) "?string $string1"
string(21) "?string|null $string2"
string(20) "?string Some Comment"
string(20) "?string Some Comment"
string(11) "?string|int"
string(16) "?string $string1"
string(21) "?string|null $string2"
string(20) "?string Some Comment"

So seems correct