docs(drand): document the meaning of "IsChained" (#11692)

chain/beacon/drand/drand.go

@@ -53,6 +53,9 @@ type DrandBeacon struct {
 	localCache *lru.Cache[uint64, *types.BeaconEntry]
 }
 
+// IsChained tells us whether this particular beacon operates in "chained mode". Prior to Drand
+// quicknet, beacons form a chain. After the introduction of quicknet, they do not, so we need to
+// change how we interact with beacon entries. (See FIP-0063)
 func (db *DrandBeacon) IsChained() bool {
 	return db.isChained
 }

node/modules/dtypes/beacon.go

@@ -13,5 +13,5 @@ type DrandConfig struct {
 	Servers       []string
 	Relays        []string
 	ChainInfoJSON string
-	IsChained     bool
+	IsChained     bool // Prior to Drand quicknet, beacons form a chain, post quicknet they do not (FIP-0063)
 }