cli(sync[docs]): Document plan config fields

why: SyncPlanConfig fields render bare in the API reference, hiding
that offline overrides fetch.

what:
- Add an Attributes section to SyncPlanConfig
- Record that fetch refreshes remote refs before status is read, and
  that offline suppresses it and plans repos with uncomparable
  remote state as updates

Author: tony

src/vcspull/cli/sync.py

@@ -86,7 +86,18 @@
 
 @dataclass
 class SyncPlanConfig:
-    """Configuration options for building sync plans."""
+    """Configuration options for building sync plans.
+
+    Attributes
+    ----------
+    fetch : bool
+        Run ``git fetch --prune`` before reading status (``--fetch``), so
+        ahead/behind counts reflect current remote refs. Ignored when
+        ``offline`` is set.
+    offline : bool
+        Plan without touching the network (``--offline``). Repositories whose
+        remote state cannot be compared are planned as updates.
+    """
 
     fetch: bool
     offline: bool