RFR: JDK-8243396 Add a brief description of argfiles to the javadoc help output

[Jonathan Gibbons]

Please review a small fix to address an omission in the command-line help
output generated by `javadoc --help`.

The text that is generated is the same as that for javac, which 
incidentally,
uses the same mechanism to expand @files.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8243396
Webrev: http://cr.openjdk.java.net/~jjg/8243396/webrev.00/