Update the javadoc man page to reflect JDK 17 changes

Several new command line options were added to `javadoc` with the JDK 17 release, however the javadoc tool spec page does not provide descriptions
`--since`
`--since-label`
`--legal-notices`

Descriptions provided from help output of command line tool.

Recent API:
```
    --since <release>(,<release>)*
                  Document new and deprecated API in the specified releases
    --since-label <text>
                  Provide text to use in the heading of the "New API" page
```

legal-notices:
```
    --legal-notices 'default' | 'none' | <directory>
                  Control legal notices in the generated output
```