make sure biosample attribute list download is in documentation

[Bradford Condon]

[Bradford Condon]

there was confusion about the base list. there is no bas eproperty list, only the organism-specific "plain" packages.

for example, the plant list:

https://www.ncbi.nlm.nih.gov/biosample/docs/packages/Plant.1.0/?format=xml

Still, would be nice to add this to the RTD somewhere...