Add note to to_device regarding copy behavior when moving to same device

[kgryte]

This PR

• closes to_device should not copy when the operation is a no op #645 by adding a note concerning expected behavior when a specified device corresponds to the device on which an array instance resides. Namely, copy behavior is left implementation-dependent, with conforming array libraries being allowed to either return self or perform an explicit copy, as discussed in to_device should not copy when the operation is a no op #645 (comment).

cc @leofang @rgommers

[rgommers]

LGTM, thanks @kgryte. This is a straightforward clarification, so can be merged as is I believe. A potential new copy= keyword can always be added later, this PR doesn't change that.

[rgommers]

In it goes then, thanks both!