doc: explain const FINALITY_CHAIN_EXTRA_EPOCHS

src/rpc/methods/chain.rs

@@ -1196,6 +1196,16 @@ impl ChainGetTipSetFinalityStatus {
             find_threshold_depth,
         };
 
+        /// Number of extra epochs to fetch beyond [`chain_finality`] when
+        /// building the chain sample for [`find_threshold_depth`].
+        ///
+        /// [`find_threshold_depth`] probes epoch depths up to `chain_finality`
+        /// using a bisect search over the sampled chain slice. The extra epochs
+        /// provide a small tail buffer so that the bisect can safely evaluate
+        /// entries at the very edge of the finality window without reading past
+        /// the end of the slice — particularly important when null rounds (epochs
+        /// with zero blocks) are present, since they consume slots in the vector
+        /// without advancing the meaningful epoch count.
         const FINALITY_CHAIN_EXTRA_EPOCHS: usize = 5;
 
         let finality = ctx.chain_config().policy.chain_finality;