8225561: jlink --help doesn't state that ALL-MODULE-PATH is accepted …

…for --add-modules

Reviewed-by: alanb

src/jdk.jlink/share/classes/jdk/tools/jlink/resources/jlink.properties

@@ -46,7 +46,8 @@ main.opt.module-path=\
 \                                        if it exists.
 
 main.opt.add-modules=\
-\      --add-modules <mod>[,<mod>...]    Root modules to resolve
+\      --add-modules <mod>[,<mod>...]    Root modules to resolve in addition to the\n\
+\                                        initial modules. <mod> can also be ALL-MODULE-PATH.
 
 main.opt.limit-modules=\
 \      --limit-modules <mod>[,<mod>...]  Limit the universe of observable\n\