RFR: JDK-8194955: Warn when default HTML version is used

[Jonathan Gibbons]

Please review a simple fix to generate a warning when the user does not
explicitly specify whether to use HTML 4.01 or HTML5. The default is
(still) HTML 4.01, but is planned to be changed in an upcoming release,
such as JDK 11.

Users should ensure that they specify the version of HTML used in
their doc comments, and which will be used for the generated output.

The change to the tool is very simple.  A new test is added that is specific
to this feature. A small number of other tests that are sensitive to the 
number
of warnings generated have been updated to avoid generating the warning.

JBS: https://bugs.openjdk.java.net/browse/JDK-8194955
Webrev: http://cr.openjdk.java.net/~jjg/8194955/webrev.00/index.html

-- Jon