gpart.8: Document one of gpart destroy diagnostic message

Reviewed by:	ziaee
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D52992

lib/geom/part/gpart.8

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd February 11, 2025
+.Dd October 24, 2025
 .Dt GPART 8
 .Os
 .Sh NAME
@@ -1497,6 +1497,26 @@ and
 .Bd -literal -offset indent
 /sbin/gpart backup ada0 | /sbin/gpart restore -F ada1 ada2
 .Ed
+.Sh DIAGNOSTICS
+.Bl -diag
+.It gpart: arg0 '%s': Invalid argument
+The provided
+.Ar geom
+argument
+is not a GEOM provider.
+Not every device in
+.Xr devfs 4
+is a GEOM provider.
+For example, a
+.Xr zfs 4
+zvol will show up as a GEOM provider only if its
+.Sy volmode
+is set properly
+.Po refer to
+.Xr zfsprops 8
+for details
+.Pc .
+.El
 .Sh SEE ALSO
 .Xr geom 4 ,
 .Xr boot0cfg 8 ,