state time window

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

docs/nightly/en/user-guide/continuous-aggregation/define-time-window.md

@@ -1,6 +1,6 @@
 # Define Time Window
 
-Time window is an important attribute of your continuous aggregation query. It defines how the data is aggregated in the flow. GreptimeDB provides two types of time windows: `hop` and `tumble`, or "sliding window" and "fixed window" in other words. You can specify the time window in the `GROUP BY` clause using `hop()` function or `tumble()` function respectively.
+Time window is an important attribute of your continuous aggregation query. It defines how the data is aggregated in the flow. GreptimeDB provides two types of time windows: `hop` and `tumble`, or "sliding window" and "fixed window" in other words. You can specify the time window in the `GROUP BY` clause using `hop()` function or `tumble()` function respectively. These two functions are only supported in continuous aggregate queries's `GROUP BY` position.
 
 # Tumble
 
@@ -14,7 +14,7 @@ Where `col` specifies use which column to compute the time window. The provided
 
 `start_time` is an optional parameter to specify the start time of the first window. If not provided, the start time will be aligned to calender.
 
-# Hop
+# Hop (not supported yet)
 
 `hop` defines sliding window that moves forward by a fixed interval. It signature is like the following:
 
@@ -24,6 +24,6 @@ hop(col, size_interval, hop_interval, <start_time>)
 
 Where `col` specifies use which column to compute the time window. The provided column must have a timestamp type.
 
-`size_interval` specifies the size of each window, while `hop_interval` specifies the delta interval between windows' start timestamp. You can think the `tumble()` function as a special case of `hop()` function where the `size_interval` and `hop_interval` are the same.
+`size_interval` specifies the size of each window, while `hop_interval` specifies the delta between two windows' start timestamp. You can think the `tumble()` function as a special case of `hop()` function where the `size_interval` and `hop_interval` are the same.
 
 `start_time` is an optional parameter to specify the start time of the first window. If not provided, the start time will be aligned to calender.