Add info that Regex.scan/3 is only matching non-overlapping matches (#13153)

With corresponding doctests.

Author: Lakret

lib/elixir/lib/regex.ex

@@ -471,8 +471,7 @@ defmodule Regex do
   end
 
   @doc ~S"""
-  Same as `run/3`, but scans the target several times collecting all
-  matches of the regular expression.
+  Same as `run/3` but returns all non-overlapping matches of the regular expression.
 
   A list of lists is returned, where each entry in the primary list represents a
   match and each entry in the secondary list represents the captured contents.
@@ -497,6 +496,12 @@ defmodule Regex do
       iex> Regex.scan(~r/e/, "abcd")
       []
 
+      iex> Regex.scan(~r/ab|bc|cd/, "abcd")
+      [["ab"], ["cd"]]
+
+      iex> Regex.scan(~r/ab|bc|cd/, "abbccd")
+      [["ab"], ["bc"], ["cd"]]
+
       iex> Regex.scan(~r/\p{Sc}/u, "$, £, and €")
       [["$"], ["£"], ["€"]]