document unique(i -> a[i], eachindex(a)) trick (JuliaLang#45291)

Co-authored-by: Milan Bouchet-Valat <nalimilan@club.fr>

Author: stevengj

base/set.jl

@@ -195,6 +195,28 @@ julia> unique(x -> x^2, [1, -1, 3, -3, 4])
  3
  4
 ```
+This functionality can also be used to extract the *indices* of the first
+occurrences of unique elements in an array:
+```jldoctest
+julia> a = [3.1, 4.2, 5.3, 3.1, 3.1, 3.1, 4.2, 1.7];
+
+julia> i = unique(i -> a[i], eachindex(a))
+4-element Vector{Int64}:
+ 1
+ 2
+ 3
+ 8
+
+julia> a[i]
+4-element Vector{Float64}:
+ 3.1
+ 4.2
+ 5.3
+ 1.7
+
+julia> a[i] == unique(a)
+true
+```
 """
 function unique(f, C; seen::Union{Nothing,Set}=nothing)
     out = Vector{eltype(C)}()