mtd: nand: reword nand_chip bad block interface comments

This remedies a few problems:

(1) The use of "the" vs. "a" is a little confusing, IMO.

(2) nand_chip.block_bad is used exclusively for checking the OOB bad
    block markers of a NAND. Any BBT functionality is handled in
    nand_bbt.c, so this description should differentiate itself from
    nand_bbt.c.
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>
Signed-off-by: NArtem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>

include/linux/mtd/nand.h

@@ -390,8 +390,8 @@ struct nand_buffers {
  * @write_buf:		[REPLACEABLE] write data from the buffer to the chip
  * @read_buf:		[REPLACEABLE] read data from the chip into the buffer
  * @select_chip:	[REPLACEABLE] select chip nr
- * @block_bad:		[REPLACEABLE] check, if the block is bad
- * @block_markbad:	[REPLACEABLE] mark the block bad
+ * @block_bad:		[REPLACEABLE] check if a block is bad, using OOB markers
+ * @block_markbad:	[REPLACEABLE] mark a block bad
  * @cmd_ctrl:		[BOARDSPECIFIC] hardwarespecific function for controlling
  *			ALE/CLE/nCE. Also used to write command and address
  * @init_size:		[BOARDSPECIFIC] hardwarespecific function for setting