Differentiating among clients

[hettling]

https://github.com/naturalis/nbaR/blob/master/vignettes/nbaR.Rmd#L474

It would be great to have some text here to explain how the taxonomic data services and TaxonClient use cases differ from the SpecimenClient use cases described earlier in the vignette. (This would be helpful regardless of whether the "Get started" vignette stays as one big document, or is split up.) Same comment for the geographic data services: https://github.com/naturalis/nbaR/blob/master/vignettes/nbaR.Rmd#L616