Clarify snapshot+targets metadata hash check

Update the client workflow to clarify that snapshot metadata hashes
can only be checked if timestamp lists the optional hashes, and,
similarly, targets metadata hashes can only be checked if snapshot
lists the optional hashes.

Author: lukpueh

tuf-spec.md

@@ -1165,10 +1165,10 @@ the timestamp metadata file.  In either case, the client MUST write the file to
 non-volatile storage as FILENAME.EXT.
 
   * **3.1**. **Check against timestamp metadata.** The hashes and version
-  number of the new snapshot metadata file MUST match the hashes and version
-  number listed in timestamp metadata.  If hashes and version do not match,
-  discard the new snapshot metadata, abort the update cycle, and report the
-  failure.
+  number of the new snapshot metadata file MUST match the hashes (if any) and
+  version number listed in the trusted timestamp metadata.  If hashes and
+  version do not match, discard the new snapshot metadata, abort the update
+  cycle, and report the failure.
 
   * **3.2**. **Check signatures.** The new snapshot metadata file MUST have
   been signed by a threshold of keys specified in the trusted root metadata
@@ -1213,11 +1213,12 @@ VERSION_NUMBER is the version number of the targets metadata file listed in the
 snapshot metadata file.  In either case, the client MUST write the file to
 non-volatile storage as FILENAME.EXT.
 
-  * **4.1**. **Check against snapshot metadata.** The hashes (if any), and
-  version number of the new targets metadata file MUST match the trusted
-  snapshot metadata.  This is done, in part, to prevent a mix-and-match attack
-  by man-in-the-middle attackers.  If the new targets metadata file does not
-  match, discard it, abort the update cycle, and report the failure.
+  * **4.1**. **Check against snapshot metadata.** The hashes and version
+  number of the new targets metadata file MUST match the hashes (if any) and
+  version number listed in the trusted snapshot metadata.  This is done, in
+  part, to prevent a mix-and-match attack by man-in-the-middle attackers.  If
+  the new targets metadata file does not match, discard it, abort the update
+  cycle, and report the failure.
 
   * **4.2**. **Check for an arbitrary software attack.** The new targets
   metadata file MUST have been signed by a threshold of keys specified in the