This would be called a prescriptive easement.

Hi Ian,In almost all states there are limits to what an unlicensed people can and cannot do, just think of the medical profession, can you prescribe prescriptions?

Your listing language is fine, and you aren't doing any of the stupid things many landlords do, like use very prescriptive language or listing lots of rules. 

While you cannot ask WHY they need the animal, you can request proof of prescription.

Mowing the lawn would give you a prescriptive easement.  

Work under someone serve coffee, and fill prescriptions then.

Because words soothe and, like a narcotic, they lull us into a state of complacency.

I am wondering about updating it to better independent senior housing with no medical/prescription services or anything like that.  

(At least in my market) You are in a more red leaning state which may have case law that is more landlord friendlyIf the dog is an Emotional Support Animal you can request a doctors note, or a prescription for the dog. 

It's common for a tenant to get a "prescription" or even multiple prescriptions so they can register all their pets as ESAs but I don't accept that.You have to decide where you are willing to put your foot down.