Make IO.getn and IO.read docs consistent (#15565)

Author: nseaSeb

lib/elixir/lib/io.ex

@@ -152,10 +152,9 @@ defmodule IO do
 
   The `device` is iterated as specified by the `line_or_chars` argument:
 
-    * if `line_or_chars` is an integer, it represents the number of characters
-      according to the device encoding (either latin1 or utf8). The device is
-      iterated by that number of characters. Use `binread/2` if you desire to
-      read bytes.
+    * if `line_or_chars` is an integer, it is the number of Unicode
+      code points to be retrieved for devices open in Unicode/utf8 mode.
+      Otherwise, it is the number of raw bytes to be retrieved.
 
     * if `line_or_chars` is `:line`, the device is iterated line by line.
       CRLF newlines  ("\r\n") are automatically normalized to "\n".
@@ -522,7 +521,7 @@ defmodule IO do
   end
 
   @doc """
-  Gets a number of bytes from IO device `:stdio`.
+  Gets a number of characters from IO device `:stdio`.
 
   If `:stdio` is a Unicode device, `count` implies
   the number of Unicode code points to be retrieved.
@@ -550,7 +549,7 @@ defmodule IO do
   end
 
   @doc """
-  Gets a number of bytes from the IO `device`.
+  Gets a number of characters from the IO `device`.
 
   If the IO `device` is a Unicode device, `count` implies
   the number of Unicode code points to be retrieved.