split statement: add a description (#11686) (#11968)

sql-statements/sql-statement-split-region.md

@@ -225,6 +225,12 @@ SPLIT TABLE t INDEX idx3 BETWEEN ("2010-01-01 00:00:00", "a") AND ("2010-01-01 0
 
 This statement splits 10 Regions in the range of a~z according to the value of column b, with the same time prefix as column a. If the value specified for column a is different, the value of column b might not be used in this case.
 
+If the primary key of the table is a [non-clustered index](/clustered-indexes.md), you need to use backticks ``` ` ``` to escape the `PRIMARY` keyword when splitting Regions. For example:
+
+```sql
+SPLIT TABLE t INDEX `PRIMARY` BETWEEN (-9223372036854775808) AND (9223372036854775807) REGIONS 16;
+```
+
 #### Uneven Split
 
 Index data can also be split by specified index values.