[12.x] Add docs about using 'exists' rule on array

[DeffiDeffi]

I recently stumbled upon this bit of (to my knowledge) undocumented behaviour of the exists rule.

I assumed that exists would not work on array values because the documentation did not specify that it would. So I validated incoming arrays of IDs like this:

'user_ids' => ['array'],
'user_ids.*' => ['integer', 'exists:users,id'],

This queries the database for every single item in the array. Instead, you can specify the 'exists' rule as:

'user_ids' => ['array', 'exists:users,id'],

... which only generates a single WHERE id IN (...) query. This PR adds a little note on this to the docs.

[shaedrich]

Good catch 👍🏻