Correct contract of Window#getShell()

Currently, Window#getShell() only states to return null if the shell has
not been created yet. It will, however, also return null if the Window
has been closed. The contract of Window#close() only states that the
shell will be disposed, but not that #getShell() will return null
afterwards as well. This change improves the explicit contract with this
information. It has been part of the implicit contract anyway, since it
is the behavior ever since the class and method exist.

Author: HeikoKlare

bundles/org.eclipse.jface/src/org/eclipse/jface/window/Window.java

@@ -655,8 +655,8 @@ public int getReturnCode() {
 	/**
 	 * Returns this window's shell.
 	 *
-	 * @return this window's shell, or <code>null</code> if this window's
-	 *         shell has not been created yet
+	 * @return this window's shell, or <code>null</code> if this window's shell has
+	 *         not been created yet or if this window has been closed
 	 */
 	@Override
 	public Shell getShell() {