improve AbstractDict doc (#34110)

matbesancon · JeffBezanson · commit ce260dc47dd9 · 2019-12-19T13:21:12.000-08:00
diff --git a/base/essentials.jl b/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
 
