website: example of "terraform taint" with a grandchild module

I've seen folks ask about how to express this in resource address syntax
a number of times now, so adding this example here to illustrate how it
looks when there are multiple levels of module to traverse through.

This is redundant with other information further up the page, but having
it as an entirely separate example gives an opportunity to include more
introductory text to explain what the example is showing.

website/docs/commands/taint.html.markdown

@@ -36,8 +36,9 @@ the case.
 Usage: `terraform taint [options] address`
 
 The `address` argument is the address of the resource to mark as tainted.
-The address is in the usual resource address syntax, as shown in
-the output from other commands, such as:
+The address is in
+[the resource address syntax](/docs/internals/resource-addressing.html) syntax,
+as shown in the output from other commands, such as:
 
  * `aws_instance.foo`
  * `aws_instance.bar[1]`
@@ -92,3 +93,14 @@ This example will only taint a resource within a module:
 $ terraform taint "module.couchbase.aws_instance.cb_node[9]"
 Resource instance module.couchbase.aws_instance.cb_node[9] has been marked as tainted.
 ```
+
+Although we recommend that most configurations use only one level of nesting
+and employ [module composition](/docs/modules/composition.html), it's possible
+to have multiple levels of nested modules. In that case the resource instance
+address must include all of the steps to the target instance, as in the
+following example:
+
+```
+$ terraform taint "module.child.module.grandchild.aws_instance.example[2]"
+Resource instance module.child.module.grandchild.aws_instance.example[2] has been marked as tainted.
+```