Help: Correct and expand AppImage CPack generator docs

The help text for most variables were just copy-and-pasted from the
minimal output from running `appimagetool --help`. That text lacked
considerable important detail.

The default values for most of the variables were also specified
incorrectly. They listed themselves as the default, which is wrong.
In most cases, the variables are unset by default. For those that
actually have fallback values, update them to specify that.

The CPACK_APPIMAGE_DESKTOP_FILE variable is not mandatory.
The code does provide fallback behavior when it isn't set. What is
mandatory though is that there is a .desktop file, either specified
explicitly, or found within the installed files. There is also a close
relationship with CPACK_PACKAGE_ICON that deserves explicitly
drawing attention to.

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