Revert 3f3358ac15. Add documentation
clarifying the misunderstanding that lead to the mistake, and make use
of the recently-added |bn_set_words|.
Change-Id: I58814bace3db3b0b44e2dfe09c44918a4710c621
Reviewed-on: https://boringssl-review.googlesource.com/8831
Reviewed-by: Adam Langley <agl@google.com>