Add a note about chaining multiple map+filter calls

lib/elixir/lib/map.ex

@@ -979,12 +979,14 @@ defmodule Map do
  `fun` receives the key and value of each of the
  elements in the map as a key-value pair.
 
- `Map.filter/2` is faster than using `map |> Enum.filter(fun) |> Enum.into(%{})`,
- as no intermediate list is being built.
-
  See also `reject/2` which discards all elements where the
  function returns a truthy value.
 
+ > Note: if you find yourself doing multiple calls to `Map.map/2`
+ > and `Map.filter/2` in a pipeline, it is likely more efficient
+ > to use `Enum.map/2` and `Enum.filter/2` instead and convert to
+ > a map at the end using `Map.new/1`.
+
  ## Examples
 
  iex> Map.filter(%{one: 1, two: 2, three: 3}, fn {_key, val} -> rem(val, 2) == 1 end)
@@ -1010,9 +1012,8 @@ defmodule Map do
  end
 
  @doc """
- Returns map excluding the pairs from `map` for which `fun` returns a truthy value.
- `Map.reject/2` is faster than using `map |> Enum.reject(fun) |> Enum.into(%{})`,
- as no intermediate list is being built.
+ Returns map excluding the pairs from `map` for which `fun` returns
+ a truthy value.
 
  See also `filter/2`.
 
@@ -1041,8 +1042,15 @@ defmodule Map do
  end
 
  @doc """
- Maps the function `fun` over all key-value pairs in `map`, returning a map
- with all the values replaced with the result of the function.
+ Maps the function `fun` over all key-value pairs in `map`
+
+ It returns a map with all the values replaced with the result
+ of the function.
+
+ > Note: if you find yourself doing multiple calls to `Map.map/2`
+ > and `Map.filter/2` in a pipeline, it is likely more efficient
+ > to use `Enum.map/2` and `Enum.filter/2` instead and convert to
+ > a map at the end using `Map.new/1`.
 
  ## Examples