Documentation: correct the description of FIEMAP_EXTENT_LAST

Currently FIEMAP_EXTENT_LAST is not working consistently across
different filesystem's fiemap implementations. So add more information
about how else this flag could set in other implementation.

Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/5a00e8d4283d6849e0b8f408c8365b31fbc1d153.1582880246.git.riteshh@linux.ibm.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>