RFR: 8309670: java -help output for --module-path / -p is incomplete

[Christian Stein]

Please review this update to `java`'s help output for `--module-path`/`-p`.

This PR also updates the output for the closely related `--upgrade-module-path` option.

Thanks in advance!

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

Commit messages:
 - Update `--upgrade-module-path` description
 - 8309670: java -help output for --module-path / -p is incomplete

Changes: https://git.openjdk.org/jdk/pull/14564/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14564&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309670
  Stats: 7 lines in 1 file changed: 2 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/14564.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14564/head:pull/14564

PR: https://git.openjdk.org/jdk/pull/14564