unhide release create --required flag, update tests, docs

[alicenstar]

Shortcut: https://app.shortcut.com/replicated/story/133212/release-create-command-should-also-support-required-flag-if-promoted-flag-used

This PR un-hides the release create --required flag, which already existed (and works), but was hidden from the docs.

# it works:
$ ./bin/replicated release create --yaml-dir ../wordpress-enterprise --app wordpress-enterprise --promote Testing --required
  • Reading manifests from ../wordpress-enterprise ✓  
  • Creating Release ✓  
    • SEQUENCE: 177
  • Promoting ✓  
    • Channel 2C0OR0393ffJXZfcFsOrNT9eJX4 successfully set to release 177

# it errors as it should:
$ ./bin/replicated release create --yaml-dir ../wordpress-enterprise --app wordpress-enterprise --required   
Error: --required can only be used with --promote <channel>

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