zpoolprops.8: clarify vdev expansion rules

Remove reference to EFI(?), explain that the new space
is beyond the GPT for whole-disk vdevs, and add section noting how it
behaves with partition vdevs in terms of how the user is most likely to
encounter it ‒ the previous phrasing was confusing
and seemed to indicate that "zpool online -e" will be able to claim

  GPT[whatever, ZFS, free space, whatever]

into

  GPT[whatever, ZFS, whatever]
but that's not the case, as it'll only be able to do so after manually
resizing the ZFS partition to include the free space beforehand, i.e.:
  GPT[whatever, ZFS, free space, whatever]
  GPT[whatever, [ZFS + free space], potentially left-overs, whatever]
  # zpool online -e
  GPT[whatever, ZFS, whatever]

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by: Ahelenia Ziemiańska <[email protected]>
Closes #11158

Author: nabijaczleweli

man/man8/zpoolprops.8

@@ -54,12 +54,15 @@ This property can also be referred to by its shortened column name,
 .It Sy expandsize
 Amount of uninitialized space within the pool or device that can be used to
 increase the total capacity of the pool.
-Uninitialized space consists of any space on an EFI labeled vdev which has not
-been brought online
-.Po e.g, using
-.Nm zpool Cm online Fl e
-.Pc .
-This space occurs when a LUN is dynamically expanded.
+On whole-disk vdevs, this is the space beyond the end of the GPT –
+typically occurring when a LUN is dynamically expanded
+or a disk replaced with a larger one.
+On partition vdevs, this is the space appended to the partition after it was
+added to the pool – most likely by resizing it in-place.
+The space can be claimed for the pool by bringing it online with
+.Sy autoexpand=on
+or using
+.Nm zpool Cm online Fl e .
 .It Sy fragmentation
 The amount of fragmentation in the pool. As the amount of space
 .Sy allocated