update configuration docs

to close node-red#25

Author: Dave Conway-Jones

docs/configuration.md

@@ -186,11 +186,19 @@ functionGlobalContext
 : Function Nodes - a collection of objects to attach to the global function
   context. For example,
 
-      functionGlobalContext: { os:require('os') }
+      functionGlobalContext: { osModule:require('os') }
 
   can be accessed in a function node as:
 
-      context.global.os
+      var myos = global.get('osModule');
+
+ <div class="doc-callout"><em>Note</em>: Prior to Node-RED v0.13, the documented
+ way to use global context was to access it as a sub-property of <code>context</code>:
+ <pre>context.global.foo = "bar";
+ var osModule = context.global.osModule;</pre>
+ This method is still supported, but deprecated in favour of the <code>global.get</code>/<code>global.set</code>
+ functions. This is in anticipation of being able to persist the context data in a future release.
+ </div>
 
 debugMaxLength
 : Debug Nodes - the maximum length, in characters, of any message sent to the