UBI: improve Kconfig documentation

	  erase counter value and the lowest erase counter value of eraseblocks

	  of UBI devices. When this threshold is exceeded, UBI starts performing

	  wear leveling by means of moving data from eraseblock with low erase

	  counter to eraseblocks with high erase counter. Leave the default

	  value if unsure.

	  counter to eraseblocks with high erase counter.

	  The default value should be OK for SLC NAND flashes, NOR flashes and

	  other flashes which have eraseblock life-cycle 100000 or more.

	  However, in case of MLC NAND flashes which typically have eraseblock

	  life-cycle less then 10000, the threshold should be lessened (e.g.,

	  to 128 or 256, although it does not have to be power of 2).

config MTD_UBI_BEB_RESERVE

	int "Percentage of reserved eraseblocks for bad eraseblocks handling"