improve documentation for SFF file loader

Link: http://permalink.gmane.org/gmane.science.biology.ray-genome-assembler/588
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
deal with the current path if the depth is not 0

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>