Documents do (x=1, y=2) -> syntax

The existing documentation explains:

> When using a JavaScript loop to generate functions, it's common to insert a closure wrapper in order to ensure that loop variables are closed over, and all the generated functions don't just share the final values. CoffeeScript provides the do keyword, which immediately invokes a passed function, forwarding any arguments.

Version 1.3.1 added the ability to use `do` to protect variables from being hoisted:

    x = 0
    do (x = 1, y = undefined) ->
      alert(x) # alerts '1'
    alert(x) # alerts '0'

This commit documents this pattern under the variables and scoping section.