[DOC] Update docs to explain the case of filtering after KNN

docs/api/sql/NearestNeighbourSearching.md

@@ -19,6 +19,30 @@ In case there are ties in the distance, the result will include all the tied geo
 spark.sedona.join.knn.includeTieBreakers=true
 ```
 
+Filter Pushdown Considerations:
+
+When using ST_KNN with filters applied to the resulting DataFrame, some of these filters may be pushed down to the object side of the kNN join. This means the filters will be applied to the object side reader before the kNN join is executed. If you want the filters to be applied after the kNN join, ensure that you first materialize the kNN join results and then apply the filters.
+
+For example, you can use the following approach:
+
+Scala Example:
+
+```
+val knnResult = knnJoinDF.cache()
+val filteredResult = knnResult.filter(condition)
+```
+
+SQL Example:
+
+```
+CREATE OR REPLACE TEMP VIEW knnResult AS
+SELECT * FROM (
+  -- Your KNN join SQL here
+) AS knnView;
+CACHE TABLE knnResult;
+SELECT * FROM knnResult WHERE condition;
+```
+
 SQL Example
 
 Suppose we have two tables `QUERIES` and `OBJECTS` with the following data: