DOC: Add Index.difference to API doc
Add `Index.difference` to api doc rather than deprecated `Index.diff`  (#8226)

And is it OK to remove `Index.diff` alias because #6581 lists #8227 (which closes #8226)?

yep no problem with changing this in the API docs (but move as @jorisvandenbossche mentions)
 The deprecation of `Index.diff` is actually not explicitely mentioned in #6581, we should add that. Also, we should clean up that list for 0.17, as not all things listed there should already be removed in 0.17
 @jorisvandenbossche yep, that list really should be for 'next-major-release' then separate section for 0.17.0
 I think it is natural under the same category as `intersection` and `union`. How about moving `difference` and `sym_diff` to "Combining / joining / merging" category, renaming it to "Combining / joining / set operations"?
 yes, sounds good to me
 agreed, making a new sub-section for setops would be gr8.
 OK, did the suggested change. Pls check.
