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	dEukaryota skHolozoa kAnimalia ks spLophotrochozoa pRotifera ps pi sc cMonogononta cs ci so oPloimida os sf f fs g__Brachionus
Propagated Ranks	dEukaryota skHolozoa kAnimalia ksAnimalia spLophotrochozoa pRotifera psRotifera piRotifera scRotifera cMonogononta csMonogononta ciMonogononta soMonogononta oPloimida osPloimida sfPloimida fPloimida fsPloimida g__Brachionus

So if a user wanted infraclass w/o rank propagation they'd get: ci With Rank propagation they'd get: ciMonogononta`

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.