update rebuild tag index doc

docs/en/12-taos-sql/07-tag-index.md

@@ -49,3 +49,5 @@ You can also add filter conditions to limit the results.
 6. You can' create index on a normal table or a child table. 
 
 7. If the unique values of a tag column are too few, it's better not to create index on such tag columns, the benefit would be very small. 
+
+8. The newly created super table will randomly generate an index name for the first column of tags, which is composed of the name tag0 column with 23 random bytes, and can be rebuilt or dropped.  

docs/zh/12-taos-sql/07-tag-index.md

@@ -48,4 +48,6 @@ SELECT * FROM information_schema.INS_INDEXES
 
 6. 不支持对普通和子表建立索引。
 
-7. 如果某个 tag 列的唯一值较少时，不建议对其建立索引，这种情况下收效甚微。
+7. 如果某个 tag 列的唯一值较少时，不建议对其建立索引，这种情况下收效甚微。
+
+8. 新建立的超级表，会给第一列tag，随机生成一个indexNewName, 生成规则是:tag0的name + 23个byte, 在系统表可以查,也可以按需要drop,行为和其他列tag 的索引一样