Clarify "paths" and "path_hash_prefixes"

Clarify "paths" and "path_hash_prefixes" in delegations, because
currently, it's not properly defined which of these options can be used
to create a valid target file:
- BOTH paths and path_hash_prefixes
- ONLY ONE of paths and path_hash_prefixes
- NONE of paths and path_hash_prefixes

With this change, I aim to define clearly that a valid target file will
contain ONLY ONE of them or NONE of them.

Signed-off-by: Martin Vrachev <[email protected]>

Author: MVrachev

tuf-spec.md

@@ -1021,9 +1021,8 @@ format:
     package that are not made by the delegated party or its descendants to be
     ignored.
 
-In order to discuss target paths, a role MUST specify only one of the
-<a>"path_hash_prefixes"</a> or <a for="delegation-role">"paths"</a> attributes, each of which we
-discuss next.
+The <a>"path_hash_prefixes"</a> and <a for="delegation-role">"paths"</a>
+attributes are OPTIONAL, if used, exactly one of them should be set.
 
   : <dfn>"path_hash_prefixes"</dfn>
   ::