add xr.set_options to docs

[raybellwaves]

#4992 (comment)

[dcherian]

Its in API reference but not sure if it's anywhere else: https://xarray.pydata.org/en/stable/generated/xarray.set_options.html display_max_rows does seem to be missing... PR?

[keewis]

it isn't anywhere else, but api.rst does contain two links to set_options

[keewis]

should have been fixed by #5000