Document that offset should return nothing if no offset was used

Author: ararslan

src/regressionmodel.jl

@@ -114,7 +114,7 @@ function reconstruct! end
     offset(model::RegressionModel)
 
 Return the offset used in the model, i.e. the term added to the linear predictor with
-known coefficient 1.
+known coefficient 1, or `nothing` if the model was not fit with an offset.
 """
 function offset end