Clarify invariant preconditions

Signed-off-by: Mihai Maruseac <[email protected]>

Author: mihaimaruseac

model_signing/manifest/manifest.py

@@ -60,7 +60,8 @@ class ManifestItem(metaclass=ABCMeta):
     def from_str(as_string: str) -> Self:
         """Parses a `ManifestItem` from a string representation.
 
-        All implementations must ensure the following round-trip properties:
+        All implementations must ensure the following round-trip properties
+        (for inputs that are valid):
 
             1) `ManifestItem.from_str(str(si)) == si`
             2) `str(ManifestItem.from_str(str)) == str`