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 <mvrachev@vmware.com>

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"