docs: we support mTLS on L4 (#4927)

Signed-off-by: spacewander <spacewanderlzx@gmail.com>

docs/en/latest/stream-proxy.md

@@ -112,6 +112,7 @@ apisix:
 
 Second, we need to configure certificate for the given SNI.
 See [Admin API's SSL section](./admin-api.md#ssl) for how to do.
+mTLS is also supported, see [Protect Route](./mtls.md#protect-route) for how to do.
 
 Third, we need to configure a stream route to match and proxy it to the upstream:
 

docs/zh/latest/stream-proxy.md

@@ -110,6 +110,7 @@ apisix:
 
 接着，我们需要为给定的 SNI 配置证书。
 具体步骤参考 [Admin API 的 SSL](./admin-api.md#ssl)。
+mTLS 也是支持的，参考 [保护路由](./mtls.md#保护路由)。
 
 然后，我们需要配置一个 route，匹配连接并代理到上游：