Improve docstring

Author: blegat

src/Bridges/Variable/vectorize.jl

@@ -2,7 +2,10 @@
     VectorizeBridge{T, S}
 
 Transforms a constrained variable in `scalar_set_type(S, T)` where
-`S <: VectorLinearSet` constrained variables in `S`.
+`S <: VectorLinearSet` into a constrained vector of one variable in `S`. For
+instance, `VectorizeBridge{Float64, MOI.Nonnegatives}` transforms a constrained
+variable in `MOI.GreaterThan{Float64}` into a constrained vector of one
+variable in `MOI.Nonnegatives`.
 """
 mutable struct VectorizeBridge{T, S} <: AbstractBridge
     variable::MOI.VariableIndex