The example at the end of the description for `-doctitle` option refers to `-header`:

> For example,`javadoc -header "<b>My Library</b><br>v1.0" com.mypackage`.

It should read `javadoc -doctitle ...` instead.

Manpage of Javadoc 14
https://docs.oracle.com/en/java/javase/14/docs/specs/man/javadoc.html#standard-doclet-options

Manpage source
https://github.com/openjdk/jdk/blob/93904460818dfbd6bf028b06777238603ce78614/src/jdk.javadoc/share/man/javadoc.1#L599