doc: add cross references in subset-related operators (#39750)

Author: AtsushiSakai

base/abstractset.jl

@@ -246,6 +246,8 @@ function ⊇ end
 
 Determine whether every element of `a` is also in `b`, using [`in`](@ref).
 
+See also [`⊊`](@ref), [`⊈`](@ref).
+
 # Examples
 ```jldoctest
 julia> issubset([1, 2], [1, 2, 3])
@@ -306,6 +308,8 @@ function ⊋ end
 
 Determines if `a` is a subset of, but not equal to, `b`.
 
+See also [`issubset`](@ref) (`⊆`), [`⊈`](@ref).
+
 # Examples
 ```jldoctest
 julia> (1, 2) ⊊ (1, 2, 3)
@@ -329,6 +333,8 @@ function ⊉ end
 
 Negation of `⊆` and `⊇`, i.e. checks that `a` is not a subset of `b`.
 
+See also [`issubset`](@ref) (`⊆`), [`⊊`](@ref).
+
 # Examples
 ```jldoctest
 julia> (1, 2) ⊈ (2, 3)