Data.Pfun naming

Stream: mathlib4

[Sky Wilshaw]

We have docs#PFun.Dom but docs#PFun.ran , I think it would be better if one were changed to match the other.

[Thomas Murrills]

I believe Sets are treated as any other type wrt naming (Set X is underlyingly X → Prop, but we don't unfold definitions before applying the naming convention rules), so I think we should change PFun.Dom → PFun.dom.

[Sky Wilshaw]

That was my instinct too. When I get time I may send in a PR (although feel free to do it if I don't get round to it).