DOC: Add Index.difference to API doc

[sinhrks]

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)?

[jorisvandenbossche]

But I think this does not belong in this section at all, as this is not a 'computation' function

I would rather put it together with the other set operation methods

[jreback]

yep no problem with changing this in the API docs (but move as @jorisvandenbossche mentions)

[jorisvandenbossche]

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

[jreback]

@jorisvandenbossche yep, that list really should be for 'next-major-release' then separate section for 0.17.0

[sinhrks]

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"?

[jorisvandenbossche]

yes, sounds good to me

[jreback]

agreed, making a new sub-section for setops would be gr8.

[sinhrks]

OK, did the suggested change. Pls check.