note infix syntax for "in" and "∈" (#54091)

I noticed that the docstring for `in` did not mention the infix syntax
up-front, making it easier to [be confused about the argument
order](https://discourse.julialang.org/t/in-x-y-and-y-in-x-behave-differently/112965).

Author: stevengj

base/operators.jl

@@ -1382,11 +1382,15 @@ a function equivalent to `y -> item in y`.
 
 Determine whether an item is in the given collection, in the sense that it is
 [`==`](@ref) to one of the values generated by iterating over the collection.
+Can equivalently be used with infix syntax:
+
+    item in collection
+    item ∈ collection
+
 Return a `Bool` value, except if `item` is [`missing`](@ref) or `collection`
 contains `missing` but not `item`, in which case `missing` is returned
 ([three-valued logic](https://en.wikipedia.org/wiki/Three-valued_logic),
 matching the behavior of [`any`](@ref) and [`==`](@ref)).
-
 Some collections follow a slightly different definition. For example,
 [`Set`](@ref)s check whether the item [`isequal`](@ref) to one of the elements;
 [`Dict`](@ref)s look for `key=>value` pairs, and the `key` is compared using