ide-assists: Clarify that generated setters receive `&mut self`

[9999years]

This is different from "builder setters", which typically receive an owned self and return Self as well.

I'd like to implement generation for fn with_foo(mut self, foo: T) -> Self methods in the future, but first we should clarify what rust-analyzer means by "setter method"s.

[ChayimFriedman2]

I don't see the point in this PR. A setter is usually a normal setter, a builder is not called a setter.