Before Java 7 the Xusage.txt file was used to print the "java -X" help information in English. From Java 7 this information was provided in localised format by the launcher. The Xusage.txt file was not removed at the time because it was also used by the gamma launcher. But the gamma launcher was removed in java 8 -
JDK-8008772. The information in Xusage.txt has become stale and incomplete (despite several edits post Java 7). It should just be removed.