[PATCH v2] fstrim: document kernel return minlen explicitly

[Wang Shilong <wangshilong1991@gmail.com>]

From: Wang Shilong <wshilong@ddn.com>

Filesystem will modify @minlen according to its
block size etc, and will return actual unit
to userspace, document it into manpage.

Cc: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
---
v1->v2:
better explain as darrick suggestion..
---
 sys-utils/fstrim.8 | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/sys-utils/fstrim.8 b/sys-utils/fstrim.8
index f7e7aa9ef..c6ecca706 100644
--- a/sys-utils/fstrim.8
+++ b/sys-utils/fstrim.8
@@ -76,10 +76,11 @@ the end of the filesystem.
 .IP "\fB\-m, \-\-minimum\fP \fIminimum-size\fP"
 Minimum contiguous free range to discard, in bytes. (This value is internally
 rounded up to a multiple of the filesystem block size.)  Free ranges smaller
-than this will be ignored.  By increasing this value, the fstrim operation
-will complete more quickly for filesystems with badly fragmented freespace,
-although not all blocks will be discarded.  The default value is zero,
-discarding every free block.
+than this will be ignored and fstrim will adjust the minimum if it's smaller than
+the device's minimum, and report that (fstrim_range.minlen) back to userspace.
+By increasing this value, the fstrim operation will complete more quickly for
+filesystems with badly fragmented freespace, although not all blocks will be
+discarded.  The default value is zero, discarding every free block.
 .IP "\fB\-v, \-\-verbose\fP"
 Verbose execution.  With this option
 .B fstrim
-- 
2.21.0