Metadata API: Add a comment for Key validation

Clarify that we don't semantically validate "Key" instances during
initialization and that this is a responsibility of securesystemslib.

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

Author: MVrachev

tuf/api/metadata.py

@@ -376,6 +376,8 @@ def bump_version(self) -> None:
 
 class Key:
     """A container class representing the public portion of a Key.
+    Please note that "Key" instances are not semanticly validated during
+    initialization. We consider this as responsibility of securesystemslib.
 
     Attributes:
         keyid: An identifier string that must uniquely identify a key within