`DataArray.nbytes` listed twice in API doc block

[zmoon]

Here: https://github.com/pydata/xarray/blame/708204370ef168f35f250ca75e666246ae1420f6/doc/api.rst#L289-L295

[zmoon]

Also, I guess it is pretty self-explanatory, but perhaps some documentation could be added, maybe at least a reference to numpy.ndarray.nbytes.

[dcherian]

See also #4842 . This would be a nice and fairly simple PR if you're up for it :)

[zmoon]

Some references:

• not listed in array API attributes
• documented in numpy
• slightly documented in dask
• slightly more documented in cupy