zpool-labelclear.8: Warn that it's destructive

If I could go back in time, I would beg Sun engineers to pick a
different name. For those of us who have not read the ZFS On-Disk
Specification pdf, it is not at all obvious that clearing a "label" is
such a bad thing.

But changing the name would be a breaking change, so at least for now
we can update the documentation.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Shelvacu <git@shelvacu.com>
Closes #18347

man/man8/zpool-labelclear.8

@@ -50,6 +50,15 @@ is a cache device, it also removes the L2ARC header
 The
 .Ar device
 must not be part of an active pool configuration.
+.Pp
+This overwrites pool metadata, making all data on the
+.Ar device
+inaccessible without specialized recovery tools.
+Unlike
+.Nm zpool Cm destroy ,
+this cannot be undone by ZFS tooling.
+Only use this when you don't care about the data on the
+.Ar device .
 .Bl -tag -width Ds
 .It Fl f
 Treat exported or foreign devices as inactive.

man/man8/zpool.8

@@ -108,7 +108,9 @@ specified.
 Destroys the given pool, freeing up any devices for other use.
 .It Xr zpool-labelclear 8
 Removes ZFS label information from the specified
-.Ar device .
+.Ar device ,
+making all data inaccessible without specialized recovery tools.
+Cannot be undone with ZFS tooling.
 .El
 .
 .Ss Virtual Devices