Add map examples to Enum.reduce/3 and Map.new/2 (#12560)

Author: maco

lib/elixir/lib/enum.ex

@@ -2478,6 +2478,9 @@ defmodule Enum do
       iex> Enum.reduce([1, 2, 3], 0, fn x, acc -> x + acc end)
       6
 
+      iex> Enum.reduce(%{a: 2, b: 3, c: 4}, 0, fn {_key, val}, acc -> acc + val end)
+      9
+
   ## Reduce as a building block
 
   Reduce (sometimes called `fold`) is a basic building block in functional

lib/elixir/lib/map.ex

@@ -235,6 +235,9 @@ defmodule Map do
       iex> Map.new([:a, :b], fn x -> {x, x} end)
       %{a: :a, b: :b}
 
+      iex> Map.new(%{a: 2, b: 3, c: 4}, fn {key, val} -> {key, val * 2} end)
+      %{a: 4, b: 6, c: 8}
+
   """
   @spec new(Enumerable.t(), (term -> {key, value})) :: map
   def new(enumerable, transform)