docs: clarify that SecretNotFound may be raised for permission errors (…

…#1231)

As noticed in #1229, `get_secret()` may raise `SecretNotFoundError` if
the secret does exist but the caller does not have permission to view
it, so make this explicit in the documentation.

ops/model.py

@@ -274,7 +274,8 @@ def get_secret(self, *, id: Optional[str] = None, label: Optional[str] = None) -
             label: Secret label if fetching by label (or updating it).
 
         Raises:
-            SecretNotFoundError: If a secret with this ID or label doesn't exist.
+            SecretNotFoundError: If a secret with this ID or label doesn't exist,
+                or the caller does not have permission to view it.
         """
         if not (id or label):
             raise TypeError('Must provide an id or label, or both')