update `to_netcdf` docstring to list support for explicit CDF5 writes

Is your feature request related to a problem?

I cannot get to_netcdf() to write files in CDF5 format as identifed by the 'ncdump -k' command.

Describe the solution you'd like

When I write a netcdf file using:

D.to_netcdf( filename )

then ask ncdump to tell me the kind of file I have,

ncdump -k filename

it returns 'netCDF-4'. Unfortunately, this file won't work in the Community Atmpshere Model (CAM), as an initial condition for example. CAM will bomb when it tries to read it. After converting the file with this command:

nccopy -k cdf5 filename cdf5_filename

the file now works in CAM. Also, the command

ncdump -k cdf5_filename

returns 'cdf5'.

I confess I don't know what the nccopy command is doing, but it seems to be needed for the file to be readable by CAM. I am looking for an option in the to_netcdf method that will explicitly write 'cdf5' files without needing to resort to the nccopy command.

Describe alternatives you've considered

Writing netcdf-4 files from xarray and converting via
nccopy -k cdf5 filename cdf5_filename

Additional context

No response

Read the current to_netcdf docstring and its documented format or encoding options first. Confirm whether explicit CDF5 writing is already supported, then update the documentation to describe that support and verify that the wording matches the resulting file format.