DOC: Add better description of rank propagation

[mikerobeson]

Need to make it clear that rank propagation will propagate taxonomy ranks for all available taxonomy, using forward fill prior to subsetting the ranks requested by the user. That is :

Accession	Taxonomy-Ranks
All Available Ranks	d__Eukaryota sk__Holozoa k__Animalia ks__ sp__Lophotrochozoa p__Rotifera ps__ pi__ sc__ c__Monogononta cs__ ci__ so__ o__Ploimida os__ sf__ f__ fs__ g__Brachionus
Propagated Ranks	d__Eukaryota sk__Holozoa k__Animalia ks__Animalia sp__Lophotrochozoa p__Rotifera ps__Rotifera pi__Rotifera sc__Rotifera c__Monogononta cs__Monogononta ci__Monogononta so__Monogononta o__Ploimida os__Ploimida sf__Ploimida f__Ploimida fs__Ploimida g__Brachionus

So if a user wanted infraclass w/o rank propagation they'd get: ci__With Rank propagation they'd get:ci__Monogononta`

There is probably a better example we can use...

[mikerobeson]

I've added a description of rank propagation under the drop menu "Rank-Propagation", just before the "Species-labels: caveat emptor!" drop menu.