Add a limitation section in expand doc & link a issue tracker for a workaround

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

docs/user/ppl/cmd/expand.rst

@@ -63,3 +63,12 @@ PPL query::
     | chen  | 32  | {"city":"Miami","state":"Florida","moveInDate":{"dateAndTime":"19010811T040333.000Z"}}    |
     | chen  | 32  | {"city":"los angeles","state":"CA","moveInDate":{"dateAndTime":"20230503T080742.000Z"}}   |
     +-------+-----+-------------------------------------------------------------------------------------------+
+
+Limitations
+============
+
+* The ``expand`` command currently only supports nested arrays. Primitive
+  fields storing arrays are not currently supported. E.g. a string field
+  storing an array of strings cannot be expanded with the current
+  implementation.
+* The ``expand`` command is only available since 3.1.0.

opensearch/src/main/java/org/opensearch/sql/opensearch/executor/OpenSearchExecutionEngine.java

@@ -234,8 +234,9 @@ private void buildResultSet(
     for (int i = 1; i <= columnCount; ++i) {
       String columnName = metaData.getColumnName(i);
       RelDataType fieldType = fieldTypes.get(i - 1);
-      // The element type of struct and array is currently set to ANY.
-      // We set them using the runtime type as a workaround.
+      // TODO: Correct this after fixing issue github.com/opensearch-project/sql/issues/3751
+      //  The element type of struct and array is currently set to ANY.
+      //  We set them using the runtime type as a workaround.
       ExprType exprType;
       if (fieldType.getSqlTypeName() == SqlTypeName.ANY) {
         if (!values.isEmpty()) {