add a notice of time_duration function to indicate some specific situations

docs/UserGuide/Operators-Functions/Aggregation.md

@@ -178,3 +178,4 @@ Result:
 |                  1677570933|
 +----------------------------+
 ```
+> Note: Returns 0 if there is only one data point, or null if the data point is null.

docs/zh/UserGuide/Operators-Functions/Aggregation.md

@@ -159,3 +159,4 @@ select time_duration(s1) from root.db.d1
 |                  1677570933|
 +----------------------------+
 ```
+> 注：若数据点只有一个，则返回0，若数据点为null，则返回null。