WAL watcher: improve comments

Clarify in the first comment that it is `watch()` that waits, and reduce
verbiage.

The second comment was slightly contradictory to the first and otherwise
didn't seem to add much, since `currentSegment` was incremented just a
few lines later.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

tsdb/wlog/watcher.go

@@ -290,8 +290,8 @@ func (w *Watcher) Run() error {
 	for !isClosed(w.quit) {
 		w.currentSegmentMetric.Set(float64(currentSegment))
 
-		// Reset the value of lastSegment each iteration, this is to avoid having to wait too long for
-		// between reads if we're reading a segment that is not the most recent segment after startup.
+		// Re-check on each iteration in case a new segment was added,
+		// because watch() will wait for notifications on the last segment.
 		_, lastSegment, err := w.firstAndLast()
 		if err != nil {
 			return fmt.Errorf("wal.Segments: %w", err)
@@ -299,8 +299,6 @@ func (w *Watcher) Run() error {
 		tail := currentSegment >= lastSegment
 
 		level.Debug(w.logger).Log("msg", "Processing segment", "currentSegment", currentSegment, "lastSegment", lastSegment)
-		// On start, after reading the existing WAL for series records, we have a pointer to what is the latest segment.
-		// On subsequent calls to this function, currentSegment will have been incremented and we should open that segment.
 		if err := w.watch(currentSegment, tail); err != nil && !errors.Is(err, ErrIgnorable) {
 			return err
 		}