subset docstring edit

[selipot]

I think it is necessary to clarify in the docstring that the id_var_name variable is expected to have the dimension "traj" or traj_dim_name. We have changed several times the conventions for the dimensions of the id and traj variables.

[selipot]

This turns out necessary for me because I was reading a local version of the version 2.01 not doing what the datasets.gdp1h() function does:

url = "https://noaa-oar-hourly-gdp-pds.s3.amazonaws.com/latest/gdp-v2.01.zarr"
ds = xr.open_dataset(url, engine="zarr", decode_times=decode_times)
ds = ds.rename_vars({"ID": "id"}).assign_coords({"id": ds.ID}).drop_vars(["ids"])