UML is not Bible. Feel free while using it.
If your diagrams get complicated, then show different aspects at different diagrams. And this is generally better.
So it is OKEY to show "register new doctor" and "manage records" in different sequence diagrams.
There is no obligation such that you have to show everthing in a single diagram.And for each diagram you should also not have to show "everthing". It is okey to filter unnecessary details: to prefer not to show them based on modeler view. Because UML is just a visual thinking and communication tool.