zpoolprops.7: document leaked

It's noted very scarcely in the code as it stands, indeed the only
actual comment on this is

  /*
   * We have finished background destroying, but there is still
   * some space left in the dp_free_dir. Transfer this leaked
   * space to the dp_leak_dir.
   */

Introduced in fbeddd60b79690b6a6ececc9b00b6014d21405aa ("Illumos 4390 -
I/O errors can corrupt space map when deleting fs/vol"),
which explains, alongside the references, that this can only happen
with a corrupted pool

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13081