RFR: 8350007: Add usage message to the javadoc executable [v3]

[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

Nizar Benalla has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - respond to feedback
 - Merge branch 'master' into javadoc-usage-message
 - update test with new usage message
 - improve javadoc executable message

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23618/files
  - new: https://git.openjdk.org/jdk/pull/23618/files/4572fe9a..3d83cbd5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23618&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23618&range=01-02

  Stats: 8289 lines in 319 files changed: 5499 ins; 1706 del; 1084 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