Add warning for using array pattern with packed arrays

[stdc105]

I've recently encountered a "papercut" when using pattern matching after String.split: the split result is a PackedStringArray that does not work with array patterns. While there is a warning in the pattern matching about the type strictness above, it might also help to add an additional warning in the "Array pattern" section as it's pretty far down in the docs, a couple screens of scroll from the warning above.

I wonder if it's also worth a proposal to add matching packed arrays with array pattern too, as they are quite similar to arrays in many ways, and definitely more comparable to StringName vs. String than float vs. int.