Clarify where LC proofs are rooted

Add more detailed LC object documentation to explain that the various
merkle proofs are relative to the beacon block's state root.
Likewise, clarify that sync committees relate to the finalized header
(not to the optimistic header, which can be a period ahead).

specs/altair/light-client/sync-protocol.md

@@ -77,9 +77,9 @@ Additional documents describe how the light client sync protocol can be used:
 
 ```python
 class LightClientBootstrap(Container):
-    # The requested beacon block header
+    # Header matching the requested beacon block root
     header: BeaconBlockHeader
-    # Current sync committee corresponding to `header`
+    # Current sync committee corresponding to `header.state_root`
     current_sync_committee: SyncCommittee
     current_sync_committee_branch: Vector[Bytes32, floorlog2(CURRENT_SYNC_COMMITTEE_INDEX)]
 ```
@@ -88,12 +88,12 @@ class LightClientBootstrap(Container):
 
 ```python
 class LightClientUpdate(Container):
-    # The beacon block header that is attested to by the sync committee
+    # Header attested to by the sync committee
     attested_header: BeaconBlockHeader
-    # Next sync committee corresponding to `attested_header`
+    # Next sync committee corresponding to `attested_header.state_root`
     next_sync_committee: SyncCommittee
     next_sync_committee_branch: Vector[Bytes32, floorlog2(NEXT_SYNC_COMMITTEE_INDEX)]
-    # The finalized beacon block header attested to by Merkle branch
+    # Finalized header corresponding to `attested_header.state_root`
     finalized_header: BeaconBlockHeader
     finality_branch: Vector[Bytes32, floorlog2(FINALIZED_ROOT_INDEX)]
     # Sync committee aggregate signature
@@ -106,9 +106,9 @@ class LightClientUpdate(Container):
 
 ```python
 class LightClientFinalityUpdate(Container):
-    # The beacon block header that is attested to by the sync committee
+    # Header attested to by the sync committee
     attested_header: BeaconBlockHeader
-    # The finalized beacon block header attested to by Merkle branch
+    # Finalized header corresponding to `attested_header.state_root`
     finalized_header: BeaconBlockHeader
     finality_branch: Vector[Bytes32, floorlog2(FINALIZED_ROOT_INDEX)]
     # Sync committee aggregate signature
@@ -121,7 +121,7 @@ class LightClientFinalityUpdate(Container):
 
 ```python
 class LightClientOptimisticUpdate(Container):
-    # The beacon block header that is attested to by the sync committee
+    # Header attested to by the sync committee
     attested_header: BeaconBlockHeader
     # Sync committee aggregate signature
     sync_aggregate: SyncAggregate
@@ -134,9 +134,9 @@ class LightClientOptimisticUpdate(Container):
 ```python
 @dataclass
 class LightClientStore(object):
-    # Beacon block header that is finalized
+    # Header that is finalized
     finalized_header: BeaconBlockHeader
-    # Sync committees corresponding to the header
+    # Sync committees corresponding to the finalized header
     current_sync_committee: SyncCommittee
     next_sync_committee: SyncCommittee
     # Best available header to switch finalized head to if we see nothing else