RFR: JDK-8243113 : Always identify version of javadoc used to generate docs

[Jonathan Gibbons]

Please review a small patch to modify the "Generated by javadoc" string 
that is
added to each HTML file generated by the standard doclet.
The change is to always include the version number, so that it is always 
available
for tools analyzing the generated docs.

The timestamp remains optional, controlled by the `-notimestamp` 
command-line
option.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8243113
Webrev: http://cr.openjdk.java.net/~jjg/8243113/webrev.00/