gsea: improvement of description

The current description is misleading as of where up- and down-regulated genes are in the list of sorted analyzed genes. It's true that genes are sorted according to diff. expression from down-regulated to up-regulated genes but the GSEA inverses this list and output results (plots) will have the up-regulated genes at the beginning and the down-regulated genes at the end.

I've changed the GSEA description to be more clear on this aspect. I also shortened it to be more precise.