Add reactor-tools dependency for javadoc generation

See gh-17128

spring-boot-project/spring-boot-docs/pom.xml

@@ -323,6 +323,11 @@
 			<artifactId>micrometer-registry-wavefront</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>io.projectreactor</groupId>
+			<artifactId>reactor-tools</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>io.projectreactor.netty</groupId>
 			<artifactId>reactor-netty</artifactId>