RFR: 8356218: [macos] Document --app-content

[Alexander Matveev]

- Added following note for `--app-content` on macOS to help and man page: `The value should be a directory with the "Resources" subdirectory (or any other directory that is valid in the "Contents" directory of the application bundle). Otherwise, jpackage may produce invalid application bundle which may fail code signing and/or notarization.`
- Added warning if `--app-content` if it points to non-standard subdirectory in "Contents" directory.
- Added test to cover warning message.

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Commit messages:
 - 8356218: [macos] Document --app-content

Changes: https://git.openjdk.org/jdk/pull/26848/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26848&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356218
  Stats: 55 lines in 6 files changed: 49 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/26848.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26848/head:pull/26848

PR: https://git.openjdk.org/jdk/pull/26848