Document limitations of pushding case as range queries

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

Author: yuancu

docs/user/ppl/functions/condition.rst

@@ -227,6 +227,16 @@ Argument type: all the supported data type, (NOTE : there is no comma before "el
 
 Return type: any
 
+Limitations
+>>>>>>>>>>>
+
+When each condition is a field comparison with a numeric literal and each result expression is a string literal, the query will be optimized as `range aggregations <https://docs.opensearch.org/latest/aggregations/bucket/range>`_ if pushdown optimization is enabled. However, this optimization has the following limitations:
+
+- Null values will not be grouped into any bucket of a range aggregation and will be ignored
+- The default ELSE clause will use the string literal ``"null"`` instead of actual NULL values
+
+To avoid these edge-case limitations, set ``plugins.calcite.pushdown.enabled`` to false.
+
 Example::
 
     os> source=accounts | eval result = case(age > 35, firstname, age < 30, lastname else employer) | fields result, firstname, lastname, age, employer

integ-test/src/test/java/org/opensearch/sql/calcite/remote/CalcitePPLCaseFunctionIT.java

@@ -458,7 +458,7 @@ public void testCaseAggWithNullValues() throws IOException {
                     + "| stats avg(age) by age_category",
                 TEST_INDEX_STATE_COUNTRY_WITH_NULL));
     verifySchema(actual, schema("avg(age)", "double"), schema("age_category", "string"));
-    // TODO: There is such discrepancy because range aggregations will ignore null values
+    // There is such discrepancy because range aggregations will ignore null values
     if (isPushdownDisabled()) {
       verifyDataRows(
           actual,