Update the description of the max-lag-millis in docs (#1388)

* Update description for max-lag-millis in command-line-flags.md

* Update command-line-flags.md

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Co-authored-by: Tim Vaillancourt <[email protected]>

Author: dontstopbelieveing

doc/command-line-flags.md

@@ -190,6 +190,8 @@ When using [Connect to replica, migrate on master](cheatsheet.md#a-connect-to-re
 
 When [`--throttle-control-replicas`](#throttle-control-replicas) is provided, throttling also considers lag on specified hosts. Lag measurements on listed hosts is done by querying `gh-ost`'s _changelog_ table, where `gh-ost` injects a heartbeat.
 
+When using on master or when `--allow-on-master` is provided, `max-lag-millis` is also considered a threshold for starting the cutover stage of the migration. If the row copy is complete and the heartbeat lag is less than `max-lag-millis` cutover phase of the migration will start. 
+
 See also: [Sub-second replication lag throttling](subsecond-lag.md)
 
 ### max-load