add annotation to some examples of list clause (#2400)

as titled

docs-2.0-zh/3.ngql-guide/5.operators/8.list.md

@@ -11,27 +11,31 @@
 ## 示例
 
 ```ngql
+# 返回连接后的列表。
 nebula> YIELD [1,2,3,4,5]+[6,7] AS myList;
 +-----------------------+
 | myList                |
 +-----------------------+
 | [1, 2, 3, 4, 5, 6, 7] |
 +-----------------------+
 
+# 返回列表中的元素数量。
 nebula> RETURN size([NULL, 1, 2]);
 +------------------+
 | size([NULL,1,2]) |
 +------------------+
 | 3                |
 +------------------+
 
+# 返回 NULL 是否存在列表中。
 nebula> RETURN NULL IN [NULL, 1];
 +--------------------+
 | (NULL IN [NULL,1]) |
 +--------------------+
 | __NULL__           |
 +--------------------+
 
+# 返回在列表[2, 3, 4, 5]中存在，且也在列表[2, 3, 8]中存在的元素，并拆分显示。
 nebula> WITH [2, 3, 4, 5] AS numberlist \
     UNWIND numberlist AS number \
     WITH number \
@@ -44,6 +48,7 @@ nebula> WITH [2, 3, 4, 5] AS numberlist \
 | 3      |
 +--------+
 
+# 返回列表中下标为 1 的元素。
 nebula> WITH ['Anne', 'John', 'Bill', 'Diane', 'Eve'] AS names RETURN names[1] AS result;
 +--------+
 | result |