UBI: fix documentation and improve readability

The "max" parameter of 'find_wl_entry()' was documented incorrectly and
it actually means the maximum possible difference from the smallest erase
counter. Rename it to "diff" instead, and amend the documentation.

Reported-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Reviewed-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>