Clarify recovery from ffwd on top-level targets

To recover from a fast-forward attack on the top-level targets
metadata, only targets and snapshot (not timestamp) metadata must
be untrusted/deleted.
Targets, because it was attacker controlled, and snapshot, because
it unwittingly might have unwittingly recorded the attacker
controlled targets version.

Author: lukpueh

tuf-spec.md

@@ -1121,9 +1121,10 @@ repo](https://github.com/theupdateframework/specification/issues).
   paper](https://ssl.engineering.nyu.edu/papers/kuppusamy-mercury-usenix-2017.pdf)
   for more details.
 
-    * **1.9.1**. **Targets recovery** If a threshold of targets keys are removed
-    from the root metadata, delete the old top-level targets, snapshot, and
-    timestamp metadata files.
+    * **1.9.1**. **Targets recovery** If a threshold of targets keys have been
+    removed in the new trusted root metadata compared to the previous trusted
+    root metadata, delete the old top-level targets and snapshot metadata
+    files.
 
     * **1.9.2**. **Snapshot recovery** If a threshold of snapshot keys have
     been removed in the new trusted root metadata compared to the previous