docstring improvements

[dcherian]

Would be good to clean up these warnings

WARNING: [numpydoc] Validation warnings while processing docstring for 'cf_xarray.bounds_to_vertices':
  SS06: Summary should fit in a single line
  PR03: Wrong parameters order. Actual: ('bounds', 'bounds_dim', 'core_dims', 'order'). Documented: ('bounds', 'bounds_dim', 'order', 'core_dims')
  SA01: See Also section not found
  EX01: No examples section found

WARNING: [numpydoc] Validation warnings while processing docstring for 'cf_xarray.vertices_to_bounds':
  SS06: Summary should fit in a single line
  RT03: Return value has no description
  SA01: See Also section not found
  EX01: No examples section found

WARNING: [numpydoc] Validation warnings while processing docstring for 'cf_xarray.shapely_to_cf':
  GL01: Docstring text (summary) should start in the line immediately after the opening quotes (not in the same line, or leaving a blank line in between)
  SA01: See Also section not found
  EX01: No examples section found

WARNING: [numpydoc] Validation warnings while processing docstring for 'cf_xarray.cf_to_shapely':
  GL01: Docstring text (summary) should start in the line immediately after the opening quotes (not in the same line, or leaving a blank line in between)
  SA01: See Also section not found
  EX01: No examples section found

WARNING: [numpydoc] Validation warnings while processing docstring for 'cf_xarray.set_options':
  GL01: Docstring text (summary) should start in the line immediately after the opening quotes (not in the same line, or leaving a blank line in between)
  ES01: No extended summary found
  PR01: Parameters {'**kwargs'} not documented
  PR02: Unknown parameters {'custom_criteria', 'warn_on_missing_variables'}
  SA01: See Also section not found