Improve AclInterface phpdoc

[VincentLanglet]

HI @derrabus @wouterj,

The following code (really simplified)

foreach ($acl->getObjectAces() as $currentIndex => $currentAce) {
    $acl->updateObjectAce($currentIndex, $mask);
 }

is reporting an issue with static analysis because, updateObjectAce expect an int, but the index of getObjectAces can be int|string. So I updated the phpdoc to restrict it to int.

[VincentLanglet]

Btw, I'm sure you don't mean it that way, but I prefer not to be pinged directly. I'm subscribed to all activities in Symfony packages, so I'll see your PR whenever I want to look for new PRs.

Sure, sorry.

[derrabus]

Thank you @VincentLanglet.