RFR: [15] JDK-8247235 doclint should permit "self-closing" tags for void elements in HTML5

[Jonathan Gibbons]

Please review a small update to doclint to permit the use of '<br/>' and 
'<hr/>',
which are legal in HTML5, per the rules here:
https://html.spec.whatwg.org/multipage/syntax.html#start-tags

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8247235
Webrev: http://cr.openjdk.java.net/~jjg/8247235/webrev.00/