DOC: fix autodoc Documenter for Accessor

[jorisvandenbossche]

See my comment here: #9322 (comment)

For the methods on dt and str I fixed the api docs (9b5a5ea), but not yet for dt and str. Ideally, this needs to be done for the docs for final 0.16

[jreback]

@jorisvandenbossche moved to 0.17.0