Document or prevent flipping axes of slice targets

@etiennerichart pointed out that strings can be flipped:
```py
>>> from curtsies import fsarray
>>> arr = fsarray(['hello', 'there!'])
>>> arr[1:3, 3:4] = 'xx'
>>> arr.dumb_display()
hello
thexe!
   x
```
Is this intended? I want to say no.
`arr[1:3, 3] = 'xx'` also works.
