*: add experimental warnings (pingcap#2832)

* *: add experimental warning

Signed-off-by: Shuaipeng Yu <jackysp@gmail.com>

* address comments

Signed-off-by: Shuaipeng Yu <jackysp@gmail.com>

* address comments

Signed-off-by: Shuaipeng Yu <jackysp@gmail.com>

* Apply suggestions from code review

Co-authored-by: Keke Yi <40977455+yikeke@users.noreply.github.com>

* update

Signed-off-by: Shuaipeng Yu <jackysp@gmail.com>

* Update post-installation-check.md

* Update ticdc-overview.md

* Apply suggestions from code review

Co-authored-by: Keke Yi <40977455+yikeke@users.noreply.github.com>

Co-authored-by: Keke Yi <40977455+yikeke@users.noreply.github.com>
Co-authored-by: yikeke <yikeke@pingcap.com>
Co-authored-by: ti-srebot <66930949+ti-srebot@users.noreply.github.com>

configure-placement-rules.md

@@ -7,6 +7,10 @@ aliases: ['/docs/dev/how-to/configure/placement-rules/']
 
 # Placement Rules
 
+> **Warning:**
+>
+> In the scenario of using TiFlash, the Placement Rules feature has been extensively tested and can be used in the production environment. Except for the scenario where TiFlash is used, using Placement Rules alone has not been extensively tested, so it is not recommended to enable this feature separately in the production environment.
+
 Placement Rules is an experimental feature of the Placement Driver (PD) introduced in v4.0. It is a replica rule system that guides PD to generate corresponding schedules for different types of data. By combining different scheduling rules, you can finely control the attributes of any continuous data range, such as the number of replicas, the storage location, the host type, whether to participate in Raft election, and whether to act as the Raft leader.
 
 ## Rule system

dashboard/dashboard-intro.md

@@ -6,6 +6,10 @@ category: how-to
 
 # TiDB Dashboard Introduction
 
+> **Warning:**
+>
+> TiDB Dashboard is still an experimental feature. It is **NOT** recommended that you use it in the production environment.
+
 TiDB Dashboard is a Web UI for monitoring, diagnosing, and managing the TiDB cluster, which is available since v4.0. It is built into the PD component and does not require an independent deployment.
 
 ![TiDB Dashboard interface](/media/dashboard/dashboard-intro.gif)

post-installation-check.md

@@ -14,6 +14,10 @@ This section describes how to check the TiDB cluster status using TiDB Dashboard
 
 ### Use TiDB Dashboard
 
+> **Note:**
+>
+> TiDB Dashboard is still an experimental feature. It is **NOT** recommended that you use it in the production environment.
+
 1. Log in to TiDB Dashboard at `${pd-ip}:${pd-port}/dashboard`. The username and password is the same as that of the TiDB `root` user. If you have modified the `root` password, enter the modified password. The password is empty by default.
 
     ![TiDB-Dashboard](/media/tiup/tidb-dashboard.png)

sql-statements/sql-statement-create-index.md

@@ -84,6 +84,10 @@ Query OK, 0 rows affected (0.31 sec)
 
 ## Expression index
 
+> **Warning:**
+>
+> Expression index is still an experimental feature. It is **NOT** recommended that you use it in the production environment.
+
 TiDB can build indexes not only on one or more columns in a table, but also on an expression. When queries involve expressions, expression indexes can speed up those queries.
 
 Take the following query as an example:

system-tables/system-table-sql-diagnosis.md

@@ -7,6 +7,10 @@ aliases: ['/docs/dev/reference/system-databases/sql-diagnosis/']
 
 # SQL Diagnosis
 
+> **Warning:**
+>
+> SQL diagnosis is still an experimental feature. It is **NOT** recommended that you use it in the production environment.
+
 SQL diagnosis is a feature introduced in TiDB v4.0. You can use this feature to locate problems in TiDB with higher efficiency. Before TiDB v4.0, you need to use different tools to obtain different information.
 
 The SQL diagnosis system has the following advantages:

ticdc/ticdc-overview.md

@@ -9,7 +9,7 @@ aliases: ['/docs/dev/reference/tools/ticdc/overview/']
 
 > **Note:**
 >
-> TiCDC is experimental. It is **not recommended** to use this feature in the production environment.
+> TiCDC is still an experimental feature. It is **NOT** recommended that you use it in the production environment.
 
 [TiCDC](https://github.com/pingcap/ticdc) is a tool for replicating the incremental data of TiDB. This tool is implemented by pulling TiKV change logs. It can restore data to a consistent state with any upstream TSO, and provides [TiCDC Open Protocol](/ticdc/ticdc-open-protocol.md) to support other systems to subscribe to data changes.
 

tidb-configuration-file.md

@@ -46,8 +46,12 @@ The TiDB configuration file supports more options than command-line parameters.
 
 ### `oom-action`
 
+> **Warning:**
+>
+> This feature is still an experimental feature, which counts the memory during the writing process. For users who want to use this feature to cancel the write operation, it is not recommended to configure it as `cancel` in the production environment.
+
 - Specifies what operation TiDB performs when a single SQL statement exceeds the memory quota specified by `mem-quota-query` and cannot be spilled over to disk.
-- Default value: `"cancel"`
+- Default value: `"log"`
 - The valid options are `"log"` and `"cancel"`. When `oom-action="log"`, it prints the log only. When `oom-action="cancel"`, it cancels the operation and outputs the log.
 
 ### `enable-streaming`

tikv-configuration-file.md

@@ -95,6 +95,10 @@ This document only describes the parameters that are not included in command-lin
 
 ## readpool.unified
 
+> **Warning:**
+>
+> Unified read pool is still an experimental feature. It is **NOT** recommended that you use it in the production environment.
+
 Configuration items related to the single thread pool serving read requests. This thread pool supersedes the original storage thread pool and coprocessor thread pool since the 4.0 version.
 
 ### `min-thread-count`