add explaination for log densitive feat (#16705)

Author: hfxsd

system-variables.md

@@ -4784,10 +4784,17 @@ SHOW WARNINGS;
 - Scope: SESSION | GLOBAL
 - Persists to cluster: Yes
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
-- Type: Boolean
+- Type: Enumeration
 - Default value: `OFF`
-- This variable controls whether to hide user information in the SQL statement being recorded into the TiDB log and slow log.
-- When you set the variable to `1`, user information is hidden. For example, if the executed SQL statement is `insert into t values (1,2)`, the statement is recorded as `insert into t values (?,?)` in the log.
+- Possible values: `OFF`, `ON`, `MARKER`
+- This variable controls whether to hide the user information in the SQL statement being recorded into the TiDB log and slow log.
+- The default value is `OFF`, which means that the user information is not processed in any way.
+- When you set the variable to `ON`, the user information is hidden. For example, if the executed SQL statement is `INSERT INTO t VALUES (1,2)`, the statement is recorded as `INSERT INTO t VALUES (?,?)` in the log.
+- When you set the variable to `MARKER`, the user information is wrapped in `‹ ›`. For example, if the executed SQL statement is `INSERT INTO t VALUES (1,2)`, the statement is recorded as `INSERT INTO t VALUES (‹1›,‹2›)` in the log. If the input has `‹`, it is escaped as `‹‹`, and `›` is escaped as `››`. Based on the marked logs, you can decide whether to desensitize the marked information when the logs are displayed.
+
+> **Warning:**
+>
+> The `MARKER` option is experimental. It is not recommended that you use it in the production environment. This feature might be changed or removed without prior notice. If you find a bug, you can report an [issue](https://github.com/pingcap/tidb/issues) on GitHub.
 
 ### tidb_regard_null_as_point <span class="version-mark">New in v5.4.0</span>