Hi James
We cannot tell you what you should put in your form, but if you have severe arthritis, you are likely to be asked at assessment how you navigate stairs whether you mention a stairlift or not. The reason they are likely to ask is that arthritis inhibits mobility and if it is in your hands - your manual dexterity. Navigating stairs shows the use of both.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems