add toSet function (#1235)

docs-2.0/3.ngql-guide/3.data-types/9.type-conversion.md

@@ -2,19 +2,14 @@
 
 Converting an expression of a given type to another type is known as type conversion.
 
-## Legacy version compatibility
-
-* nGQL 1.0 adopts the `C`-style of type conversion (implicitly or explicitly): `(type_name)expression`. For example, the results of `YIELD (int)(TRUE)` is `1`. But it is error-prone to users who are not familiar with the C language.
-
-* since nGQL 2.0, the openCypher way of type coercions is adopted.
-
 ## Type coercions functions
 
 | Function    | Description                                                     |
 | -           | -                                                               |
 | toBoolean() | Converts a string value to a boolean value.                     |
 | toFloat()   | Converts an integer or string value to a floating point number. |
 | toInteger() | Converts a floating point or string value to an integer value.  |
+| toSet()     | Converts a list or set value to a set value.|
 | type()      | Returns the string representation of the relationship type.     |
 
 ## Examples
@@ -72,4 +67,17 @@ nebula> MATCH (n:player) \
 +-------+
 | 2     |
 +-------+
+
+nebula> RETURN toSet(list[1,2,3,1,2]) AS list2set;
++-----------+
+| list2set  |
++-----------+
+| {3, 1, 2} |
++-----------+
+nebula> RETURN toSet(set{1,2,3,1,2}) AS set2set;
++-----------+
+| set2set   |
++-----------+
+| {3, 2, 1} |
++-----------+
 ```