INT: followup to Index not a sub-class of ndarray
followup from #7891 
- [x] tests for `argmin/argmax`, #7951
- [x] doc-strings for properties / functions that are ndarray compat, #7891, #7951
  - [x] `any/all` doc-strings (series/index) (unbox already does this)
- [ ] `searchsorted`, #7447
- [x] `Index.__nonzero__` should raise ? #7897, fixed in #7951
- [x] see if can figure out `sortorder` (e.g. in MultiIndex.repeat), leaving for now
- [x] defined `nbytes` (for index its ok), for NDFrame this should actually return the memory of data + index memory (#6852), done in #7891
