tracker: add comments for Match and Next fields

Signed-off-by: Pavel Kalinnikov <[email protected]>

tracker/progress.go

@@ -28,7 +28,11 @@ import (
 // strewn around `*raft.raft`. Additionally, some fields are only used when in a
 // certain State. All of this isn't ideal.
 type Progress struct {
-	Match, Next uint64
+	// Match is log index up to which the follower's log matches the leader's.
+	Match uint64
+	// Next is the index of the next log entry to be sent to the follower. Entries
+	// in the (Match, Next) interval are on the fly to this follower.
+	Next uint64
 	// State defines how the leader should interact with the follower.
 	//
 	// When in StateProbe, leader sends at most one replication message