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

Documentation/filesystems/fiemap.txt

@@ -115,8 +115,10 @@ data. Note that the opposite is not true - it would be valid for
 FIEMAP_EXTENT_NOT_ALIGNED to appear alone.
 
 * FIEMAP_EXTENT_LAST
-This is the last extent in the file. A mapping attempt past this
-extent will return nothing.
+This is generally the last extent in the file. A mapping attempt past
+this extent may return nothing. Some implementations set this flag to
+indicate this extent is the last one in the range queried by the user
+(via fiemap->fm_length).
 
 * FIEMAP_EXTENT_UNKNOWN
 The location of this extent is currently unknown. This may indicate