Metada API: document timestamp.snapshot_meta

Document snapshot_meta as the API that should be used when interacting
with timestamp meta information about snapshot.
Also, document that "meta" is an internal detail mimicking the spec
file format.

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

Author: MVrachev

tuf/api/metadata.py

@@ -849,8 +849,11 @@ class Timestamp(Signed):
     Timestamp contains information about the snapshot Metadata file.
 
     Attributes:
-        meta: A dictionary of filenames to MetaFiles. The only valid key value
-            is the snapshot filename, as defined by the specification.
+        snapshot_meta: MetaFile instance used as API for interacting with
+            snapshot meta information.
+        meta: Implementation detail mimicking the spec file format. A dictionary
+            of filenames to MetaFiles. The only valid key value is the snapshot
+            filename, as defined by the specification.
     """
 
     _signed_type = "timestamp"