Add docs and fix equals / hashCode

Author: carlosdelest

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/fulltext/FullTextFunction.java

@@ -389,7 +389,14 @@ public EvalOperator.ExpressionEvaluator.Factory toEvaluator(ToEvaluator toEvalua
         return new LuceneQueryExpressionEvaluator.Factory(shardConfigs);
     }
 
+    /**
+     * Returns the query builder to be used when the function cannot be pushed down to Lucene, but uses a
+     * {@link org.elasticsearch.compute.lucene.LuceneQueryEvaluator} instead
+     *
+     * @return the query builder to be used in the {@link org.elasticsearch.compute.lucene.LuceneQueryEvaluator}
+     */
     protected QueryBuilder evaluatorQueryBuilder() {
+        // Use the same query builder as for the translation by default
         return queryBuilder();
     }
 

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/vector/Knn.java

@@ -292,7 +292,8 @@ private Map<String, Object> queryOptions() throws InvalidArgumentException {
     }
 
     protected QueryBuilder evaluatorQueryBuilder() {
-        // Either we couldn't push down due to non-pushable filters, or becauses it's part of a disjuncion. Use exact query.
+        // Either we couldn't push down due to non-pushable filters, or because it's part of a disjuncion.
+        // Uses a nearest neighbors exact query instead of an approximate one
         var fieldAttribute = Match.fieldAsFieldAttribute(field());
         Check.notNull(fieldAttribute, "Knn must have a field attribute as the first argument");
         String fieldName = getNameFromFieldAttribute(fieldAttribute);
@@ -359,13 +360,12 @@ public boolean equals(Object o) {
         return Objects.equals(field(), knn.field())
             && Objects.equals(query(), knn.query())
             && Objects.equals(queryBuilder(), knn.queryBuilder())
-            && Objects.equals(k(), knn.k())
             && Objects.equals(filterExpressions(), knn.filterExpressions());
     }
 
     @Override
     public int hashCode() {
-        return Objects.hash(field(), query(), queryBuilder(), k(), filterExpressions());
+        return Objects.hash(field(), query(), queryBuilder(), filterExpressions());
     }
 
 }