Update 3.alter-tag.md (#2025)

docs-2.0/3.ngql-guide/10.tag-statements/3.alter-tag.md

@@ -10,7 +10,7 @@
 
 - The property name must be unique in a tag. If you add a property with the same name as an existing property or a dropped property, the operation fails.
 
-- Only the length of a `FIXED_STRING` or an `INT` can be increased.
+- Only the length of a `FIXED_STRING` or an `INT` can be increased. The length of a `STRING` or an `INT` cannot be decreased.
 
 - Only the data type conversions from FIXED_STRING to STRING and from FLOAT to DOUBLE are allowed.
 
@@ -42,12 +42,12 @@ ttl_definition:
 
 ```ngql
 nebula> CREATE TAG IF NOT EXISTS t1 (p1 string, p2 int);
-nebula> ALTER TAG t1 ADD (p3 int, p4 string);
+nebula> ALTER TAG t1 ADD (p3 int32, fixed_string(10));
 nebula> ALTER TAG t1 TTL_DURATION = 2, TTL_COL = "p2";
 nebula> ALTER TAG t1 COMMENT = 'test1';
 nebula> ALTER TAG t1 ADD (p5 double NOT NULL DEFAULT 0.4 COMMENT 'p5') COMMENT='test2';
-// Change the `p3` property to INT8 
-nebula> ALTER TAG t1 CHANGE (p3 int8, p4 string);
+// Change the data type of p3 in the TAG t1 from INT32 to INT64, and that of p4 from FIXED_STRING(10) to STRING.
+nebula> ALTER TAG t1 CHANGE (p3 int64, p4 string);
 [ERROR(-1005)]: Unsupported!
 ```