doc: add note about reduce([hv]cat, x) in [hv]cat (#44049)

base/abstractarray.jl

@@ -1759,7 +1759,8 @@ end
 """
  vcat(A...)
 
-Concatenate along dimension 1.
+Concatenate along dimension 1. To efficiently concatenate a large vector of arrays,
+use `reduce(vcat, x)`.
 
 # Examples
 ```jldoctest
@@ -1785,13 +1786,29 @@ julia> vcat(c...)
 2×3 Matrix{Int64}:
  1 2 3
  4 5 6
+
+julia> vs = [[1, 2], [3, 4], [5, 6]]
+3-element Vector{Vector{Int64}}:
+ [1, 2]
+ [3, 4]
+ [5, 6]
+
+julia> reduce(vcat, vs)
+6-element Vector{Int64}:
+ 1
+ 2
+ 3
+ 4
+ 5
+ 6
 ```
 """
 vcat(X...) = cat(X...; dims=Val(1))
 """
  hcat(A...)
 
-Concatenate along dimension 2.
+Concatenate along dimension 2. To efficiently concatenate a large vector of arrays,
+use `reduce(hcat, x)`.
 
 # Examples
 ```jldoctest
@@ -1836,6 +1853,17 @@ julia> hcat(x, [1; 2; 3])
  1
  2
  3
+
+julia> vs = [[1, 2], [3, 4], [5, 6]]
+3-element Vector{Vector{Int64}}:
+ [1, 2]
+ [3, 4]
+ [5, 6]
+
+julia> reduce(hcat, vs)
+2×3 Matrix{Int64}:
+ 1 3 5
+ 2 4 6
 ```
 """
 hcat(X...) = cat(X...; dims=Val(2))