docs: add doc for force fill

docs/en/12-taos-sql/12-distinguished.md

@@ -75,6 +75,16 @@ These pseudocolumns occur after the aggregation clause.
 5. LINEAR：Fill with the closest non-NULL value, `FILL(LINEAR)`
 6. NEXT：Fill with the next non-NULL value, `FILL(NEXT)`
 
+In the above filling modes, except for `NONE` mode, the `fill` clause will be ignored if there is no data in the defined time range, i.e. no data would be filled and the query result would be empty. This behavior is reasonable when the filling mode is `PREV`, `NEXT`, `LINEAR`, because filling can't be performed if there is not any data. For filling modes `NULL` and `VALUE`, however, filling can be performed even though there is not any data, filling or not depends on the choice of user's application.  To accomplish the need of this force filling behavior and not break the behavior of existing filling modes, TDengine added two new filling modes since version 3.0.3.0. 
+
+1. NULL_F: Fill `NULL` by force
+2. VALUE_F: Fill `VALUE` by force
+
+The detailed beaviors of `NULL`, `NULL_F`, `VALUE`, and VALUE_F are described below：
+- When used with `INTERVAL`: `NULL_F` and `VALUE_F` are filling by force；`NULL` and `VALUE`  don't fill by force. The behavior of each filling mode is exactly same as what the name suggests.
+- When used with `INTERVAL` in stream processing: `NULL_F` and `NULL` are same, i.e. don't fill by force; `VALUE_F` and `VALUE` and same, i.e. don't fill by force. It's suggested that there is no filling by force in stream processing.
+- When used with `INTERP`: `NULL` and `NULL_F` and same, i.e. filling by force; `VALUE` and `VALUE_F` are same, i.e. filling by force. It's suggested that there is always filling by force when used with `INTERP`.
+
 :::info
 
 1. A huge volume of interpolation output may be returned using `FILL`, so it's recommended to specify the time range when using `FILL`. The maximum number of interpolation values that can be returned in a single query is 10,000,000.

docs/zh/12-taos-sql/12-distinguished.md

@@ -69,6 +69,16 @@ FILL 语句指定某一窗口区间数据缺失的情况下的填充模式。填
 5. LINEAR 填充：根据前后距离最近的非 NULL 值做线性插值填充。例如：FILL(LINEAR)。
 6. NEXT 填充：使用下一个非 NULL 值填充数据。例如：FILL(NEXT)。
 
+以上填充模式中，除了 NONE 模式默认不填充值之外，其他模式在查询的整个时间范围内如果没有数据 FILL 子句将被忽略，即不产生填充数据，查询结果为空。这种行为在部分模式（PREV、NEXT、LINEAR）下具有合理性，因为在这些模式下没有数据意味着无法产生填充数值。而对另外一些模式（NULL、VALUE）来说，理论上是可以产生填充数值的，至于需不需要输出填充数值，取决于应用的需求。所以为了满足这类需要强制填充数据或 NULL 的应用的需求，同时不破坏现有填充模式的行为兼容性，从 3.0.3.0 版本开始，增加了两种新的填充模式：
+
+7. NULL_F: 强制填充 NULL 值 
+8. VALUE_F: 强制填充 VALUE 值
+
+NULL, NULL_F, VALUE, VALUE_F 这几种填充模式针对不同场景区别如下：
+- INTERVAL 子句： NULL_F, VALUE_F 为强制填充模式；NULL, VALUE 为非强制模式。在这种模式下下各自的语义与名称相符
+- 流计算中的 INTERVAL 子句：NULL_F 与 NULL 行为相同，均为非强制模式；VALUE_F 与 VALUE 行为相同，均为非强制模式。即流计算中的 INTERVAL 没有强制模式
+- INTERP 子句：NULL 与 NULL_F 行为相同，均为强制模式；VALUE 与 VALUE_F 行为相同，均为强制模式。即 INTERP 中没有非强制模式。
+
 :::info
 
 1. 使用 FILL 语句的时候可能生成大量的填充输出，务必指定查询的时间区间。针对每次查询，系统可返回不超过 1 千万条具有插值的结果。