Clarify some details of global objects implementation.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 5e57bedb9ae8310a91839dc88116ffabdd528261..3b0e376d9188cbd379e4a05660f482dc757dccfd 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -3053,7 +3053,18 @@ class V8EXPORT ExtensionConfiguration {
  */
 class V8EXPORT Context {
  public:
-  /** Returns the global object of the context. */
+  /**
+   * Returns the global proxy object.
+   *
+   * Global proxy object is a thin wrapper whose prototype points to
+   * actual context's global object with the properties like Object, etc.
+   * This is done that way for security reasons (for more details see
+   * https://wiki.mozilla.org/Gecko:SplitWindow).
+   *
+   * Please note that changes to global proxy object prototype most probably
+   * would break VM---v8 expects only global object as a prototype of
+   * global proxy object.
+   */
   Local<Object> Global();
 
   /**