Xarray passes the label indexers to the underlying pandas index:

```python import pandas as pd

"x" coordinate index

idx = pd.Index([2, 1, 0, 3, 5])

da.sel(x=slice(2, 3)) does this:

idx.slice_indexer(2, 3)

which returns slice(0, 4, None)

da.sel(x=[2, 3]) does this:

idx.get_indexer([2, 3])

which returns array([0, 3])

```

What might be possible is to warn if selecting with a slice and the index is not monotonic increasing or decreasing.

Is it always desirable? Asked differently, are there cases where one intentionally wants to select with a slice a non monotonic index? If yes, a warning might be annoying.

Maybe this could be clarified in the docs too?