Document redis3 username and keyPrefix options

Filer-Redis-Setup.md

@@ -30,4 +30,19 @@ Compared to `redis2`, there are extra Redis operations to maintain this list:
 One Redis operation costs about 25 microseconds. The extra Redis operations cost about 4 Redis round trips when running with 1 million items. The cost is relatively small compared to the whole file creation/update/deletion process. 
 
 # Note
-The file read operation is still just one Redis operation, since it does not need to read the list of other directory items.
+The file read operation is still just one Redis operation, since it does not need to read the list of other directory items.
+# `redis3` configuration
+
+`redis3`, `redis_cluster3`, and `redis3_sentinel` accept the same `username`, `password`, and `keyPrefix` options as `redis2`:
+
+```toml
+[redis3]
+enabled = true
+address = "localhost:6379"
+username = "seaweedfs"   # Redis 6+/Valkey ACL user; requires `password`
+password = "SOME_PASSWORD"
+database = 0
+keyPrefix = "seaweedfs_" # every key the store writes is prefixed
+```
+
+With `keyPrefix` set, all keys — entries, directory lists, skiplist nodes, locks, and Kv — go under the prefix, so an ACL user can be restricted to `~seaweedfs_*`. A `username` without a `password` is rejected at startup, since an empty password sends no AUTH and would silently connect as the `default` user.