So, IOW, and the answer to my question is yes, it should be insured that all pages involved are physically adjacent (by design or by pre-sort) before creating a BRIN on them. Further to the point, it is self defeating to have more than one BRIN index on the table if the columns involved would have mutually non-adjacent pages. Therefore, it actually would be good to state that in the documentation, even it were just a comment.
BRIN indexes are best used on INSERT only tables with a sequence of numbers as a PK or indexed column that will be queried against. At least as I understand it.
"Further to the point, it is self defeating to have more than one BRIN index on the table if the columns involved would have mutually non-adjacent pages."
Not really, if both columns are ordered, BRIN will work
"Therefore, it actually would be good to state that in the documentation, even it were just a comment."
It is = "BRIN is designed for handling very large tables in which certain columns have some natural correlation with their physical location within the table"