resource_control: RU info in explain analyze

sql-statements/sql-statement-explain-analyze.md

@@ -282,6 +282,18 @@ commit_txn: {prewrite:48.564544ms, wait_prewrite_binlog:47.821579, get_commit_ts
 - `write_keys`: The total `keys` written in the transaction.
 - `write_byte`: The total bytes of `key-value` written in the transaction, and the unit is byte.
 
+### RU (Request Unit) Consumption
+
+[Request Unit (RU)](/tidb-resource-control.md#what-is-request-unit-ru) a unified abstraction unit of system resources which is defined in TiDB resource control. The `execution info` of top level operator shows the overall RU consumption of this particular SQL statement.
+
+```
+RU:273.842670
+```
+
+> **Notice :**
+>
+> This value shows the actual RU produced by this call. The RU consumption could vary if the same SQL statement repeats, as the amount of RU is affected by cache layer (for example, [coprocessor cache](/coprocessor-cache.md)).
+
 ### Other common execution information
 
 The Coprocessor operators usually contain two parts of execution time information: `cop_task` and `tikv_task`. `cop_task` is the time recorded by TiDB, and it is from the moment that the request is sent to the server to the moment that the response is received. `tikv_task` is the time recorded by TiKV Coprocessor itself. If there is much difference between the two, it might indicate that the time spent waiting for the response is too long, or the time spent on gRPC or network is too long.

tidb-resource-control.md

@@ -86,6 +86,10 @@ Request Unit (RU) is a unified abstraction unit in TiDB for system resources, wh
 > - In addition to queries executed by users, RU can be consumed by background tasks, such as automatic statistics collection.
 > - The preceding table lists only the resources involved in RU calculation for TiDB Self-Hosted clusters, excluding the network and storage consumption. For TiDB Serverless RUs, see [TiDB Serverless Pricing Details](https://www.pingcap.com/tidb-cloud-serverless-pricing-details/).
 
+### Predict RU consumption of SQL statements
+
+[`explain analyze`](/sql-statements/sql-statement-explain-analyze.md#ru-request-unit-consumption) shows the actual RU produced by one particular SQL statement. Please be aware the RU consumption could vary if the same SQL statement repeats, as the amount of RU is affected by cache layer (for example, [coprocessor cache](/coprocessor-cache.md)). Even though this value doesn't reflect the exact RU produced by each execution, we can still come with an estimation based on this value. 
+
 ## Parameters for resource control
 
 The resource control feature introduces two new global variables.