Improve docs for flatten

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

Author: yuancu

docs/user/ppl/cmd/appendcol.rst

@@ -11,8 +11,6 @@ appendcol
 
 Description
 ============
-    (Experimental)
-    (From 3.1.0)
 
 Use ``appendcol`` command to append the result of a sub-search and attach it alongside with the input search results (The main search).
 

docs/user/ppl/cmd/flatten.rst

@@ -35,7 +35,7 @@ flatten <field> [as (<alias-list>)]
   ``country, state, city`` and ``(country, state, city)`` are supported,
   but the latter is advised. Its length must match the number of keys in the
   struct field.  Please note that the provided alias names **must** follow
-  the alphabetical order of the corresponding original keys in the struct.
+  the lexicographical order of the corresponding original keys in the struct.
 
 Example: flatten a struct field with aliases
 ============================================
@@ -98,3 +98,16 @@ PPL query::
     | {"info":"b","author":"f","dayOfWeek":2} | 2      | f       | 2   | b    |
     +-----------------------------------------+--------+---------+-----+------+
 
+Limitations
+===========
+* The command works only with Calcite enabled. This can be set with the
+  following command:
+
+  .. code-block::
+
+    PUT /_cluster/settings
+    {
+      "persistent":{
+          "plugins.calcite.enabled": true
+      }
+    }

ppl/src/test/java/org/opensearch/sql/ppl/calcite/CalcitePPLFlattenTest.java

@@ -32,6 +32,9 @@
 import org.junit.Assert;
 import org.junit.Test;
 
+/**
+ * Unit tests for {@code flatten} command in PPL.
+ */
 public class CalcitePPLFlattenTest extends CalcitePPLAbstractTest {
   public CalcitePPLFlattenTest() {
     super(CalciteAssert.SchemaSpec.SCOTT_WITH_TEMPORAL);