Duplicate the javadoc configuration in the <reporting> section

Unfortunately, at least for a vast range of versions, mvn site:site
doesn't honor the javadoc configuration specified in the
<pluginManagement/> section.

Let's be rather safe than sorry and just duplicate it for now.

See http://jira.codehaus.org/browse/MSITE-443.

[Documentation fixups by Curtis Rueden.]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>