Add clear and highly visible information of important data properties

[pablo-gar]

There are two important data properties that all users of the Census need to be aware of, a lack of understanding on these may lead to unexpected numerical behaviors for the user:

1. Clear and visible messaging about the existence of full-length sequencing (includes smart-seq2)
2. Similarly, clear and visible messaging for duplicated data (is_primary_data == False)

There needs to be clear messaging with the maximum visibility in the documentation page of the Census

[pablo-gar]

Done #421