Add broadcasting examples to in and ∉ docstrings.

base/operators.jl

@@ -1081,6 +1081,14 @@ is always a `Bool` and never `missing`.
 To determine whether an item is not in a given collection, see [`:∉`](@ref).
 You may also negate the `in` by doing `!(a in b)` which is logically similar to "not in".
 
+When broadcasting with `in.(items, collection)` or `items .∈ collection`, both
+`item` and `collection` are broadcasted over, which is often not what is intended.
+For example, if both arguments are vectors (and the dimensions match), the result is
+a vector indicating whether each value in collection `items` is `in` the value at the
+corresponding position in `collection`. To get a vector indicating whether each value
+in `items` is in `collection`, wrap `collection` in a tuple or a `Ref` like this:
+`in.(items, Ref(collection))` or `items .∈ Ref(collection)`.
+
 # Examples
 ```jldoctest
 julia> a = 1:3:20
@@ -1109,6 +1117,16 @@ true
 
 julia> !(19 in a)
 false
+
+julia> [1, 2] .∈ [2, 3]
+2-element BitArray{1}:
+ 0
+ 0
+
+julia> [1, 2] .∈ ([2, 3],)
+2-element BitArray{1}:
+ 0
+ 1
 ```
 """
 in, ∋
@@ -1119,13 +1137,31 @@ in, ∋
 
 Negation of `∈` and `∋`, i.e. checks that `item` is not in `collection`.
 
+When broadcasting with `items .∉ collection`, both `item` and `collection` are
+broadcasted over, which is often not what is intended. For example, if both arguments
+are vectors (and the dimensions match), the result is a vector indicating whether
+each value in collection `items` is not in the value at the corresponding position
+in `collection`. To get a vector indicating whether each value in `items` is not in
+`collection`, wrap `collection` in a tuple or a `Ref` like this:
+`items .∉ Ref(collection)`.
+
 # Examples
 ```jldoctest
 julia> 1 ∉ 2:4
 true
 
 julia> 1 ∉ 1:3
 false
+
+julia> [1, 2] .∉ [2, 3]
+2-element BitArray{1}:
+ 1
+ 1
+
+julia> [1, 2] .∉ ([2, 3],)
+2-element BitArray{1}:
+ 1
+ 0
 ```
 """
 ∉, ∌