kgo.EpochOffset: export Less, add docs

Closes #192.

pkg/kgo/consumer_group.go

@@ -1637,11 +1637,23 @@ type uncommit struct {
 // EpochOffset combines a record offset with the leader epoch the broker
 // was at when the record was written.
 type EpochOffset struct {
-	Epoch  int32
+	// Epoch is the leader epoch of the record being committed. Truncation
+	// detection relies on the epoch of the CURRENT record. For truncation
+	// detection, the client asks "what is the the end of this epoch?",
+	// which returns one after the end offset (see the next field, and
+	// check the docs on kmsg.OffsetForLeaderEpochRequest).
+	Epoch int32
+
+	// Offset is the offset of a record. If committing, this should be one
+	// AFTER a record's offset. Clients start consuming at the offset that
+	// is committed.
 	Offset int64
 }
 
-func (e EpochOffset) less(o EpochOffset) bool {
+// Less returns whether the this EpochOffset is less than another. This is less
+// than the other if this one's epoch is less, or the epoch's are equal and
+// this one's offset is less.
+func (e EpochOffset) Less(o EpochOffset) bool {
 	return e.Epoch < o.Epoch || e.Epoch == o.Epoch && e.Offset < o.Offset
 }
 
@@ -1876,7 +1888,7 @@ func (g *groupConsumer) updateCommitted(
 			//
 			// w.r.t. leader epoch's, we document that modifying
 			// leader epoch's is not recommended.
-			if uncommit.head.less(set) {
+			if uncommit.head.Less(set) {
 				uncommit.head = set
 			}
 
@@ -2205,7 +2217,7 @@ func (cl *Client) MarkCommitRecords(rs ...*Record) {
 		if newHead := (EpochOffset{
 			r.LeaderEpoch,
 			r.Offset + 1,
-		}); current.head.less(newHead) {
+		}); current.head.Less(newHead) {
 			curPartitions[r.Partition] = uncommit{
 				dirty:     current.dirty,
 				committed: current.committed,