Document interpolation in object keys

Fixes #3962.

documentation/coffee/interpolation.coffee

@@ -3,7 +3,5 @@ quote  = "A picture is a fact. -- #{ author }"
 
 sentence = "#{ 22 / 7 } is a decent approximation of π"
 
-
-
-
-
+direction = "top"
+window.scrollBy "#{direction}": 100

documentation/index.html.js

@@ -887,7 +887,8 @@ Expressions
       <b class="header">String Interpolation, Block Strings, and Block Comments</b>
       Ruby-style string interpolation is included in CoffeeScript. Double-quoted
       strings allow for interpolated values, using <tt>#{ ... }</tt>,
-      and single-quoted strings are literal.
+      and single-quoted strings are literal. You may even use interpolation in
+      object keys.
     </p>
     <%= codeFor('interpolation', 'sentence') %>
     <p>