Fix javadoc-creation errors

Minor javadoc edits to fix build failure, where on previous master, 'mvn install' reports
```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:2.8.1:jar (attach-sources) on project kalium-jni: MavenReportException: Error while creating archive:
[ERROR] Exit code: 1 - /build/kalium-jni/src/main/java/org/abstractj/kalium/crypto/Random.java:29: warning: no description for @return
[ERROR] /build/kalium-jni/src/main/java/org/abstractj/kalium/encoders/Hex.java:22: error: self-closing element not allowed
```