Update description to tidb_scatter_region (#6924) (#7809)

system-variables.md

@@ -1208,6 +1208,7 @@ SET tidb_query_log_max_len = 20
 - Scope: GLOBAL
 - Default value: `OFF`
 - By default, Regions are split for a new table when it is being created in TiDB. After this variable is enabled, the newly split Regions are scattered immediately during the execution of the `CREATE TABLE` statement. This applies to the scenario where data need to be written in batches right after the tables are created in batches, because the newly split Regions can be scattered in TiKV beforehand and do not have to wait to be scheduled by PD. To ensure the continuous stability of writing data in batches, the `CREATE TABLE` statement returns success only after the Regions are successfully scattered. This makes the statement's execution time multiple times longer than that when you disable this variable.
+- Note that if `SHARD_ROW_ID_BITS` and `PRE_SPLIT_REGIONS` have been set when a table is created, the specified number of Regions are evenly split after the table creation.
 
 ### tidb_skip_ascii_check <span class="version-mark">New in v5.0</span>