Add the primary reference for the polymer_micelle model to the model help

[smk78]

As detailed in #420 there are a number of issues with the polymer_micelle model. One of these issues would be easily mitigated by simply adding the primary reference (below) for the model to the model help (since this is where the requirement that R >> Rg is actually stipulated; it is not mentioned in the existing 2000 reference!).

https://pubs.acs.org/doi/full/10.1021/ma9512115

Some explanation for this requirement could also be added to the model description.