Improve comments on sample timestamp validation errors (#8018)

This has come up time and time again, I have tried to write it up a bit
better in code first, I would also strongly suggest that we improve the
error messaging and the documentation of this.

pkg/validation/validate.go

@@ -31,8 +31,17 @@ const (
 	// StreamRateLimit is a reason for discarding lines when the streams own rate limit is hit
 	// rather than the overall ingestion rate limit.
 	StreamRateLimit = "per_stream_rate_limit"
-	OutOfOrder      = "out_of_order"
-	TooFarBehind    = "too_far_behind"
+	// OutOfOrder is a reason for discarding lines when Loki doesn't accept out
+	// of order log lines (parameter `-ingester.unordered-writes` is set to
+	// `false`) and the lines in question are older than the newest line in the
+	// stream.
+	OutOfOrder = "out_of_order"
+	// TooFarBehind is a reason for discarding lines when Loki accepts
+	// unordered ingest  (parameter `-ingester.unordered-writes` is set to
+	// `true`, which is the default) and the lines in question are older than
+	// half of `-ingester.max-chunk-age` compared to the newest line in the
+	// stream.
+	TooFarBehind = "too_far_behind"
 	// GreaterThanMaxSampleAge is a reason for discarding log lines which are older than the current time - `reject_old_samples_max_age`
 	GreaterThanMaxSampleAge         = "greater_than_max_sample_age"
 	GreaterThanMaxSampleAgeErrorMsg = "entry for stream '%s' has timestamp too old: %v, oldest acceptable timestamp is: %v"