Better document/test user-made GWCS input

I recently implemented some better handling for GWCS input in the 2.0 dev branch, mainly to support better reading of JWST data. But this should also help with manually creating a Spectrum1D with a user-created GWCS as input for the `wcs` argument, so I should add some docs/tests for that use case.

Review the improved GWCS input handling in the 2.0 development branch and inspect how a user-created GWCS is passed to Spectrum1D through the wcs argument. Add documentation and tests for manually creating a Spectrum1D with that input, and verify that the documented use case is covered by the tests.

Written by the indexing model from the issue text.