doc: add comment explaining historical epoch height calculation bug (#4785)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: envestcc

action/protocol/poll/slasher.go

@@ -363,7 +363,9 @@ func (sh *Slasher) GetProbationList(ctx context.Context, sr protocol.StateReader
 	if err != nil {
 		return nil, uint64(0), err
 	}
-	// make sure it's epochStartHeight
+	// NOTE: GetEpochHeight(GetEpochHeight(targetHeight)) is a historical bug that should have been
+	// GetEpochHeight(GetEpochNum(targetHeight)). It is kept as-is to ensure historical blocks can
+	// pass validation. This does not affect current blocks since the input is already epochStartHeight.
 	targetEpochStartHeight := rp.GetEpochHeight(rp.GetEpochHeight(targetHeight))
 	if readFromNext {
 		targetEpochNum := rp.GetEpochNum(targetEpochStartHeight) + 1