mtd: nand: expand description of read_page method in comment header

In the absence of any formal documentation of the nand interface, I thought this
patch to the header file might be helpful.

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>