Document that fluent findBy(…) queries must return a result.

Closes #3294

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/JpaSpecificationExecutor.java

@@ -217,6 +217,10 @@ public interface JpaSpecificationExecutor<T> {
 	/**
 	 * Returns entities matching the given {@link Specification} applying the {@code queryFunction} that defines the query
 	 * and its result type.
+	 * <p>
+	 * The query object used with {@code queryFunction} is only valid inside the {@code findBy(…)} method call. This
+	 * requires the query function to return a query result and not the {@link FluentQuery} object itself to ensure the
+	 * query is executed inside the {@code findBy(…)} method.
 	 *
 	 * @param spec must not be null.
 	 * @param queryFunction the query function defining projection, sorting, and the result type