[skip e2e] Add comment for getMinTick (#14751)

Signed-off-by: dragondriver <jiquan.long@zilliz.com>

internal/proxy/channels_time_ticker.go

@@ -37,6 +37,7 @@ type channelsTimeTicker interface {
 	getLastTick(pchan pChan) (Timestamp, error)
 	// getMinTsStatistics returns the last write timestamp of all pchans.
 	getMinTsStatistics() (map[pChan]Timestamp, Timestamp, error)
+	// getMinTick returns the minimum last write timestamp between all pchans.
 	getMinTick() Timestamp
 }