Admin UI: document maintenance.erasure_coding max_shard_transfers

Admin-UI.md

@@ -168,6 +168,7 @@ preferred_tags = ["fast", "ssd"]
 replica_placement = ""
 strict_placement = false
 free_slots_deadband = 0  # nodes within M free volume slots tie; fewer live shards wins
+max_shard_transfers = 4  # shard copy concurrency: unset = one in-flight per dest disk; 0 = sequential; N = global cap
 ```
 
 Keys set in `admin.toml` are applied every time the admin server starts and override values saved from the admin UI; keys left out keep their saved or default values. This makes the settings survive data directory loss, such as a recreated PVC. Requires `-dataDir`.