Clarify docstring commets about Mercury paper

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

Author: MVrachev

tuf/repository_tool.py

@@ -2919,14 +2919,14 @@ def create_new_repository(repository_directory, repository_name='default',
       Whether to include the optional length attribute for targets
       metadata files in the snapshot metadata.
       Default is False because of bandwidth considerations.
-      Read more here:
+      Read more at section 5.6 from the Mercury paper:
       https://www.usenix.org/conference/atc17/technical-sessions/presentation/kuppusamy
 
     use_snapshot_hashes:
       Whether to include the optional hashes attribute for targets
       metadata files in the snapshot metadata.
       Default is False because of bandwidth considerations.
-      Read more here:
+      Read more at section 5.6 from the Mercury paper:
       https://www.usenix.org/conference/atc17/technical-sessions/presentation/kuppusamy
 
   <Exceptions>
@@ -3026,14 +3026,14 @@ def load_repository(repository_directory, repository_name='default',
       Whether to include the optional length attribute for targets
       metadata files in the snapshot metadata.
       Default is False because of bandwidth considerations.
-      Read more here:
+      Read more at section 5.6 from the Mercury paper:
       https://www.usenix.org/conference/atc17/technical-sessions/presentation/kuppusamy
 
     use_snapshot_hashes:
       Whether to include the optional hashes attribute for targets
       metadata files in the snapshot metadata.
       Default is False because of bandwidth considerations.
-      Read more here:
+      Read more at section 5.6 from the Mercury paper:
       https://www.usenix.org/conference/atc17/technical-sessions/presentation/kuppusamy
 
   <Exceptions>