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>

include/linux/mtd/bbm.h

@@ -113,7 +113,11 @@ struct nand_bbt_descr {
 /* Do not store flash based bad block table in OOB area; store it in-band */
 #define NAND_BBT_NO_OOB		0x00040000
 
-/* The nand_bbt_descr was created dynamicaly and must be freed */
+/*
+ * Flag set by nand_create_default_bbt_descr(), marking that the nand_bbt_descr
+ * was allocated dynamicaly and must be freed in nand_release(). Has no meaning
+ * in nand_chip.bbt_options.
+ */
 #define NAND_BBT_DYNAMICSTRUCT	0x80000000
 
 /* The maximum number of blocks to scan for a bbt */