Fix javadoc search in Java 11

Closes gh-711

buildSrc/src/main/java/org/springframework/gradle/docs/SpringJavadocOptionsPlugin.java

@@ -30,6 +30,8 @@ public class SpringJavadocOptionsPlugin implements Plugin<Project> {
 		project.getTasks().withType(Javadoc.class, (javadoc) -> {
 			StandardJavadocDocletOptions options = (StandardJavadocDocletOptions) javadoc.getOptions();
 			options.addStringOption("Xdoclint:none", "-quiet");
+			// Workaround for Java 11 javadoc search bug. Can be removed with Java 17.
+			options.addBooleanOption("-no-module-directories", true);
 		});
 	}
 }