Clarify download privs on /doc/faq

[mccalluc]

Currently it reads:

Those interested in downloading available data will need to create an account within the data portal. Note that downloads of specific datasets will be anonymous. Downloads of these data sets require NIH approval and are therefore not anonymous.

• This is self-contradictory: "specific datasets will be anonymous ... these data sets ... are therefore not anonymous"
• It doesn't explain that only hubmap users get any benefit from logging in via globus.