improve AbstractDict doc (#34110)

Author: matbesancon

base/essentials.jl

@@ -19,6 +19,7 @@ abstract type AbstractSet{T} end
 
 Supertype for dictionary-like types with keys of type `K` and values of type `V`.
 [`Dict`](@ref), [`IdDict`](@ref) and other types are subtypes of this.
+An `AbstractDict{K, V}` should be an iterator of `Pair{K, V}`.
 """
 abstract type AbstractDict{K,V} end