Add RU calcuation to explain docs

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

@@ -294,6 +294,32 @@ RU:273.842670
 >
 > This value shows the actual RUs consumed by this execution. The same SQL statement might consume different amounts of RUs each time it is executed due to the effects of caching (for example, [coprocessor cache](/coprocessor-cache.md)).
 
+The RU count can be calculated from the other values in `EXPLAIN ANALYZE`; specifically, the `executeInfo` block. For example, given:
+
+```
+ 'executeInfo': 'time:2.55ms, loops:2, RU:0.329460, Get:{num_rpc:1, '
+                'total_time:2.13ms}, total_process_time: 231.5µs, '
+                'total_wait_time: 732.9µs, tikv_wall_time: 995.8µs, '
+                'scan_detail: {total_process_keys: 1, total_process_keys_size: '
+                '150, total_keys: 1, get_snapshot_time: 691.7µs, rocksdb: '
+                '{block: {cache_hit_count: 2, read_count: 1, read_byte: 8.19 '
+                'KB, read_time: 10.3µs}}}',
+```
+
+The base costs are in [the pd source](https://github.com/tikv/pd/blob/aeb259335644d65a97285d7e62b38e7e43c6ddca/client/resource_group/controller/config.go#L58C19-L67) and the calculations are done in [model.go](https://github.com/tikv/pd/blob/54219d649fb4c8834cd94362a63988f3c074d33e/client/resource_group/controller/model.go#L107)
+
+If you are using 7.1.x, the calculation is sum `BeforeKVRequest() + AfterKVRequest()` in `pd/pd-client/model.go`, that is:
+
+```
+before :
+      consumption.RRU += float64(kc.ReadBaseCost) -> kv.ReadBaseCost * rpc_nums
+after:
+      consumption.RRU += float64(kc.ReadBytesCost) * readBytes -> kc.ReadBytesCost * total_process_keys_size
+      consumption.RRU += float64(kc.CPUMsCost) * kvCPUMs -> kc.CPUMsCost * total_process_time
+```
+
+For writes & batch gets the calculation is similar with different base values.
+
 ### 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.