feat: add changelog

Author: gitmkn

.changelog/3172.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+resource/tencentcloud_mysql_readonly_instance: modify doc.
+```

website/docs/r/mysql_readonly_instance.html.markdown

@@ -96,7 +96,7 @@ The following arguments are supported:
 * `fast_upgrade` - (Optional, Int) Specify whether to enable fast upgrade when upgrade instance spec, available value: `1` - enabled, `0` - disabled.
 * `force_delete` - (Optional, Bool) Indicate whether to delete instance directly or not. Default is `false`. If set true, the instance will be deleted instead of staying recycle bin. Note: only works for `PREPAID` instance. When the main mysql instance set true, this para of the readonly mysql instance will not take effect.
 * `intranet_port` - (Optional, Int) Public access port. Valid value ranges: [1024~65535]. The default value is `3306`.
-* `master_region` - (Optional, String) The zone information of the primary instance is required when you purchase a disaster recovery instance.
+* `master_region` - (Optional, String) The region information of the master instance. This field is required when purchasing a cross-region subscription.
 * `param_template_id` - (Optional, Int) Specify parameter template id.
 * `pay_type` - (Optional, Int, **Deprecated**) It has been deprecated from version 1.36.0. Please use `charge_type` instead. Pay type of instance. Valid values: `0`, `1`. `0`: prepaid, `1`: postpaid.
 * `period` - (Optional, Int, **Deprecated**) It has been deprecated from version 1.36.0. Please use `prepaid_period` instead. Period of instance. NOTES: Only supported prepaid instance.