readelf: document that -u / --unwind is not yet implemented

ELF tool chain readelf accepts -u / --unwind but just ignores the
option. This was previously undocumented, which could be confusing for
someone encountering readelf -u (in a script or GNU readelf example).

Reported by: markj (in D32003)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit deef4b8ce8ba7292fe5088bf9f6d4e2e35662fe8)