Document Map.fetch!/2 exception in its doc (#14573)

Author: PragTob

lib/elixir/lib/map.ex

@@ -317,6 +317,11 @@ defmodule Map do
       iex> Map.fetch!(%{a: 1}, :a)
       1
 
+  When the key is missing, an exception is raised:
+      
+      Map.fetch!(%{a: 1}, :b)
+      ** (KeyError) key :b not found in: %{a: 1}
+
   """
   @spec fetch!(map, key) :: value
   def fetch!(map, key) do