[DOC] Correct active device requirements for deallocation of `device_buffer`, `device_vector` and friends

[wence-]

Report incorrect documentation

Since #1370, it is no longer necessary for the user to ensure that the currently active device matches the device used during allocation for a device_buffer when the destructor runs. However, we still say that one must in the README documentation.

Conversely, since the initialized device_vector is just a wrapper around a thrust::device_vector, those objects do still have the previous restriction that the active device during deallocation must be the same as the active device during allocation (and this must be arranged by the user).

[harrism]

Aside: @jrhemstad has suggested that maybe RMM should remove the rmm::device_vector alias, because of inconsistencies like this...

[wence-]

I think the last time we went through this (#1370 (comment)) we decided (based on usages throughout rapids) that we wouldn't remove it.