RFR [9] 8076224: some tidy warning from core libs

[alexander stepanov]

Hello,

Could you please review the following fix
http://cr.openjdk.java.net/~avstepan/8076224/webrev.00/

for
https://bugs.openjdk.java.net/browse/JDK-8076224

Just some HTML cleanup for docs (remove tidy warnings; remove some 
unnecessary <tt> tags as they are not supported in HTML5).

Thanks,
Alexander