mtd: nand: improve comment on NAND_BBT_DYNAMIC_STRUCT

In an attempt to improve the documentation of the BBT code, I am expanding
the comments I left in commit:
    58373ff0
    mtd: nand: more BB Detection refactoring and dynamic scan options

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>