- Fix documentation for --aircraft-dir: the specified path is
interpreted relatively to the current directory, not to the path of
the fgfs executable.
- Add a few precisions (the option bypasses the <path-cache> from
autosave_X_Y.xml, as well as --fg-aircraft and FG_AIRCRAFT).
- Show the option documentation in 'fgfs --help -v' output (it was
"hidden" since 2010, but it works very well; cf.
<https://sourceforge.net/p/flightgear/mailman/message/34494887/> for
the most recent discussion about this option).
- Show the doc for --aircraft-dir right below that of --aircraft (both
being closely related).
- Add a pointer to --aircraft-dir in --fg-aircraft's doc, since the
former can be used as an alternative to the latter (or at least a
complement, since --fg-aircraft is used to determine some
permissions).
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