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

Closes #3237

src/main/java/org/springframework/data/querydsl/QuerydslPredicateExecutor.java

@@ -115,6 +115,10 @@ public interface QuerydslPredicateExecutor<T> {
 	/**
 	 * Returns entities matching the given {@link Predicate} applying the {@link Function 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 predicate must not be {@literal null}.
 	 * @param queryFunction the query function defining projection, sorting, and the result type

src/main/java/org/springframework/data/querydsl/ReactiveQuerydslPredicateExecutor.java

@@ -136,6 +136,10 @@ public interface ReactiveQuerydslPredicateExecutor<T> {
 	/**
 	 * Returns entities matching the given {@link Predicate} applying the {@link Function 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 predicate must not be {@literal null}.
 	 * @param queryFunction the query function defining projection, sorting, and the result type

src/main/java/org/springframework/data/repository/query/QueryByExampleExecutor.java

@@ -95,6 +95,10 @@ public interface QueryByExampleExecutor<T> {
 	/**
 	 * Returns entities matching the given {@link Example} applying the {@link Function 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 example must not be {@literal null}.
 	 * @param queryFunction the query function defining projection, sorting, and the result type

src/main/java/org/springframework/data/repository/query/ReactiveQueryByExampleExecutor.java

@@ -84,6 +84,10 @@ public interface ReactiveQueryByExampleExecutor<T> {
 	/**
 	 * Returns entities matching the given {@link Example} applying the {@link Function 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 example must not be {@literal null}.
 	 * @param queryFunction the query function defining projection, sorting, and the result type