trim.8: minor update for manual page

Further explain that trim(8) is not for trimming free blocks
in populated file systems/ZFS pools, as people still take it wrong sometimes.

MFc after:	1 week

usr.sbin/trim/trim.8

@@ -23,7 +23,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd October 1, 2025
+.Dd December 9, 2025
 .Dt TRIM 8
 .Os
 .Sh NAME
@@ -51,11 +51,21 @@ utility erases specified region of the device.
 It is mostly relevant for a storage that implements trim (like flash based,
 or thinly provisioned storage).
 .Sy All erased data is lost.
-Alternatively, refer to the
+.Pp
+The
+.Nm
+utility should not be used upon mounted file system or active devices
+being a part of imported ZFS pool or other software RAID, and the kernel
+does not permit such usage generally.
+If used upon not mounted file system
+of inactive device, the
+.Nm
+utility erases blocks non-selectively, whether a block is occupied or not.
+Refer to the
 .Xr fsck_ffs 8
 command having a flag
 .Fl E
-to trim populated UFS, and to the
+to trim free blocks inside populated UFS, and to the
 .Xr zpool-trim 8
 command to trim existing ZFS pool.
 .Pp