This commit improves how the build deals with javadoc invalid references in two ways.
Link/see references that are temporarily invalid during javadoc
generation of individual modules are better masked by using the option
Xdoclint:syntax instead of Xdoclint:none (warnings were still
visible in some cases, e.g. when individually building the javadoc for
a specific module).
Global javadoc-building task api now combines syntax and reference
Xdoclint groups, allowing to raise truly invalid references even when
all the modules have been aggregated.
This commit also fixes the 20+ errors which appeared following the later change in doclet configuration.