Update nand.h

Don't merge, just notes
Somewhere page used as block alias, according to http://www.eng.umd.edu/~blj/CS-590.26/micron-tn2919.pdf block is minimal eraseable thing, page is minimal readable/writeable thing.
We are going to run on stm32f10x later, where individual words (u32) could be read/written, and erased could be performed on 1Kibs