Document and test "unpacking" of compound models in fitters

See https://github.com/astropy/specutils/pull/210#discussion_r215767298 . I'm not sure how best to document this - perhaps as a worked example in the just-merged #210 fitting narrative docs?

Start with the discussion in astropy/specutils#210 and the fitting narrative documentation it references. Determine how compound-model unpacking is expected to work, then document it as a worked example and add tests for that behavior. Done means the narrative explains the workflow and the tests verify unpacking.

Written by the indexing model from the issue text.