docstring improvements

Search for the docstrings of cf_xarray.bounds_to_vertices, vertices_to_bounds, shapely_to_cf, cf_to_shapely, and set_options, then compare them with the listed numpydoc warnings. Update the summaries, parameter and return documentation, See Also sections, and examples as applicable, and confirm that the reported validation warnings are resolved.

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