Add toString in type convertion function table

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

@@ -9,6 +9,7 @@ Converting an expression of a given type to another type is known as type conver
 | 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.  |
+| toString()  | Converts non-compound types of data, such as numbers, booleans, and so on, to strings.|
 | toSet()     | Converts a list or set value to a set value.|
 | type()      | Returns the string representation of the relationship type.     |
 
@@ -68,12 +69,20 @@ nebula> MATCH (n:player) \
 | 2     |
 +-------+
 
+nebula> RETURN toString(9669) AS int2str, toString(null) AS null2str;
++---------+----------+
+| int2str | null2str |
++---------+----------+
+| "9669"  | __NULL__ |
++---------+----------+
+
 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   |