Improve docs for IEx.Helpers.open

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: José Valim

lib/iex/lib/iex/helpers.ex

@@ -240,19 +240,24 @@ defmodule IEx.Helpers do
 
       subl path/to/file:line
 
+  It is important that you choose an editor command that does
+  not block nor that attempts to run an editor directly in the
+  terminal. Command-line based editors likely extra configuration
+  so they open up the given file and line in a separate window.
+
   Custom editors are supported by using the __FILE__ and __LINE__
-  notations. For example, vi/vim users can set `ELIXIR_EDITOR` to:
+  notations, for example:
 
-      ELIXIR_EDITOR="vi +__LINE__ __FILE__"
+      ELIXIR_EDITOR="my_editor +__LINE__ __FILE__"
 
   and Elixir will properly interpolate values.
 
   Since this function prints the result returned by the editor,
   `ELIXIR_EDITOR` can be set "echo" if you prefer to display the
   location rather than opening it.
 
-  Keep in mind the location may not exist when opening
-  precompiled source code, such as Elixir itself.
+  Keep in mind the location may not exist when opening precompiled
+  source code.
 
   ## Examples