Make <wbr> element void

According to https://html.spec.whatwg.org/multipage/syntax.html#elements-2 <wbr> is a void element. A void element does not have an end tag. However, jdk.javadoc.internal.doclint.HtmlTag.WBR is modelled as an element with a required end tag.

Although it doesn't seem to cause any problems if inserted by the standard doclet (e.g. JDK-8266779), a <wbr> inserted in a doc comment causes an unwarranted doclint error "element not closed: wbr".