ADDITIONAL SYSTEM INFORMATION :
macOS 10.14.6 + updates
OpenJDK 13

A DESCRIPTION OF THE PROBLEM :
The online Javadoc documentation includes the unrecognized option -Xmodule:

It should be removed from the documentation.

See https://docs.oracle.com/en/java/javase/13/docs/specs/man/javadoc.html

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
javadoc -Xmodule:xyz

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Not in documentation
ACTUAL -
In documentation

FREQUENCY : always