update documentation, and minor fix for MultisetRelDataType

docs/dev/table/sql.md

@@ -746,6 +746,7 @@ The SQL runtime is built on top of Flink's DataSet and DataStream APIs. Internal
 | `Types.PRIMITIVE_ARRAY`| `ARRAY`                     | e.g. `int[]`           |
 | `Types.OBJECT_ARRAY`   | `ARRAY`                     | e.g. `java.lang.Byte[]`|
 | `Types.MAP`            | `MAP`                       | `java.util.HashMap`    |
+| `Types.MULTISET`       | `MULTISET`                  | `java.util.HashMap`    |
 
 
 Advanced types such as generic types, composite types (e.g. POJOs or Tuples), and array types (object or primitive arrays) can be fields of a row.
@@ -2107,6 +2108,17 @@ VAR_SAMP(value)
         <p>Returns the sample variance (square of the sample standard deviation) of the numeric field across all input values.</p>
       </td>
     </tr>
+
+    <tr>
+      <td>
+          {% highlight text %}
+          COLLECT(value)
+          {% endhighlight %}
+      </td>
+      <td>
+          <p>Returns a multiset of the <i>value</i>s.</p>
+      </td>
+    </tr>
   </tbody>
 </table>
 
@@ -2226,7 +2238,6 @@ The following functions are not supported yet:
 
 - Binary string operators and functions
 - System functions
-- Collection functions
 - Distinct aggregate functions like COUNT DISTINCT
 
 {% top %}

flink-libraries/flink-table/src/main/scala/org/apache/flink/table/plan/schema/MultisetRelDataType.scala

@@ -30,7 +30,7 @@ class MultisetRelDataType(
     elementType,
     isNullable) {
 
-  override def toString = s"MULTISET($typeInfo)"
+  override def toString = s"MULTISET($elementType)"
 
   def canEqual(other: Any): Boolean = other.isInstanceOf[MultisetRelDataType]