RFR: 8350007: Add usage message to the javadoc executable

[Nizar Benalla]

This patch adds a new message when you run the `javadoc` executable with any arguments.
Currently, unlike most other tools, running `javadoc` without any arguments does not show you how to use the tool or point you to use the `--help` option, this can be improved.

Before change:

W $ ./javadoc
error: No modules, packages or classes specified.
1 error


After change:

W $ ./javadoc
Usage:
    javadoc [options] [packagenames] [sourcefiles] [@files]
For additional help on usage:           javadoc --help

-------------

Commit messages:
 - improve javadoc executable message

Changes: https://git.openjdk.org/jdk/pull/23618/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23618&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350007
  Stats: 9 lines in 2 files changed: 4 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/23618.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23618/head:pull/23618

PR: https://git.openjdk.org/jdk/pull/23618