Update C.83 with swap for resource mgmt. (#1694)

Update C.83 with swap for resource mgmt. (#1694)

* Update C.83 with swap for resource mgmt.

The swap is useful to implement assignments idiomatically (e.g. copy-swap idiom).

With the current enforcement, to non-virtual classes, very simple classes (e.g. `trivially_copyable`, a struct encapsulating an stl container) are suggested to have a swap. This suggestion can be argued wrong since for very simple classes copy-swap idiom is not efficient (the creation of a third object on assignment is not needed, which copy-swap does).

* Rework C.83 according to PR comments.