API: raise on setops for + and - for Indexes (GH8226)
closes #8226 
- ToDo
  - [x] currently a raise `TypeError` (need a better message), possibly could deprecate and just warn
  - [x]  API example

Combining indexes via `+` now requires `.union()`
Diffing indexes via `-` now requires `.diff()`

These make the Indexes use setops (`&`,`|`,`^`) ONLY
numeric ops will work on appropriate Index ONLY (see #8184) (e.g. `DatetimeIndex` +/- `Timedelta` or `TimedeltaIndex` +/- `TImedeltaIndex`)
Also `Int64/Float64Index` would gain ability to `+/-` as well 

This will promote consistency that `+` means add (and not union), and `-` means subtract (and not difference), which IIRC has come up in a few issues
