Add docs for enable-tcp4-only (#5169)

tidb-configuration-file.md

@@ -155,6 +155,12 @@ The TiDB configuration file supports more options than command-line parameters.
 - Default value: `true`
 - When this configuration is set to `false` on all TiDB instances, the telemetry collection in TiDB is disabled and the [`tidb_enable_telemetry`](/system-variables.md#tidb_enable_telemetry-new-in-v402-version) system variable does not take effect. See [Telemetry](/telemetry.md) for details.
 
+### `enable-tcp4-only` <span class="version-mark">New in v5.0.0</span>
+
+- Enables or disables listening on TCP4 only.
+- Default value: `false`
+- Enabling this option is useful when TiDB is used with LVS for load balancing because the [real client IP from the TCP header](https://github.com/alibaba/LVS/tree/master/kernel/net/toa) can be correctly parsed by the "tcp4" protocol.
+
 ## Log
 
 Configuration items related to log.
@@ -594,4 +600,4 @@ The `experimental` section, introduced in v3.1.0, describes configurations relat
 ### `allow-expression-index` <span class="version-mark">New in v4.0.0</span>
 
 - Determines whether to create the expression index.
-- Default value: `false`
+- Default value: `false`