archive: document that --add-virtual-file takes full path

archive: document that --add-virtual-file takes full path

Tom Scogland noticed that `--add-virtual-file` option uses the path
specified as its value as-is, without prepending any value given to
the `--prefix` option like `--add-file` does.

The behaviour has always been that way since the option was
introduced, but the documentation has always been wrong and said
that it would use the value of `--prefix` just like `--add-file`
does.

We could modify the behaviour to make it literally work like the
documentation said, but it would break existing scripts the users
use.

Noticed-by: Tom Scogland <scogland1@llnl.gov>
Acked-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>