DataTree: selecting via [["a"]] suggests subset, which is missing

[mathause]

What happened?

calling dt[["a"]] suggests to use .subset which does not exist.

What did you expect to happen?

No response

Minimal Complete Verifiable Example

import xarray as xr
a = xr.Dataset(data_vars={"x": [10, 20]}, coords={"time": [0, 1]})

dt = xr.DataTree()
dt["a"] = a

dt[["a"]] # fails and suggests subset
dt.subset # fails

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

Anything else we need to know?

No response

Environment

master