Code review comment for ~anj/epics-base/+git/base-7.0:fix-1824277

Looking at this again. What concerns me is that dbPutFieldLink() sits at the conjunction of three external interfaces (rset, dset, and lset). It isn't clear to me what the expectations and allowed actions are. Particularly of link support. If this is clear to anyone (Andrew) I would find it quite helpful to see some comments added to describe the state(s) that the target link may be in at, and after, the external interface calls.

As an example. I've just noticed that dbDbRemoveLink() always free()s the memory pointed to be 'plink->value.pv_link.pvt', but doesn't always NULL the pointer.