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

@@ -1327,9 +1327,10 @@ it in the next step.
   paper](https://ssl.engineering.nyu.edu/papers/kuppusamy-mercury-usenix-2017.pdf)
   for more details.
 
-    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. **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.
 
     2. **Snapshot recovery** If a threshold of snapshot keys have
     been removed in the new trusted root metadata compared to the previous