Minor/javadoc warnings as errors (#171)

build.gradle

@@ -207,6 +207,11 @@ allprojects {
   // Normalise Xdoclint behaviour across JDKs (OpenJDK 8 is more lenient than Oracle JDK by default).
   javadoc {
     options.addStringOption('Xdoclint:all', '-quiet')
+    if (JavaVersion.current() == JavaVersion.VERSION_1_8) {
+      options.addStringOption('Xwerror', '-quiet')
+    } else {
+      options.addStringOption('Xwerror', '-html5')
+    }
     options.encoding = 'UTF-8'
   }