Warn against erasing array elements while iterating.

Erasing array elements while iterating does not appear to be safe.
For example, the following prints nothing:

```
var a := [0,1,2,3,4,5,6]
for i in a:
        if i % 2 == 0:
                a.erase(i)
        else:
                print(i)
```

While this is often true for array implementations, it still seems worth
documenting explicitly. I copied the wording from Dictionary.xml.

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