virt-win-reg: Document syntax for deleting keys and values (RHBZ#737944).

NB: This requires hivex >= 1.3.2 before this syntax becomes true.

tools/virt-win-reg

@@ -640,6 +640,17 @@ to find out is to look at the C<HKLM\SYSTEM\Select> key:
 Similarly, other C<Current...> keys in the path may need to
 be replaced.
 
+=head1 DELETING REGISTRY KEYS AND VALUES
+
+To delete a whole registry key, use the syntax:
+
+ [-HKEY_LOCAL_MACHINE\Foo]
+
+To delete a single value within a key, use the syntax:
+
+ [HKEY_LOCAL_MACHINE\Foo]
+ "Value"=-
+
 =head1 WINDOWS TIPS
 
 Note that some of these tips modify the guest disk image.  The guest