virsh: Fix man page argument ordering for vol-download command

The proper command order is 'virsh vol-download volume file pool'. While
making the modification clean up the description a bit too in order
to help clarify under what circumstances the volume could be found
if the pool name was not provided.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

tools/virsh.pod

@@ -4104,19 +4104,28 @@ See the description for the libvirt virStorageVolUpload API for details
 regarding possible target volume and pool changes as a result of the
 pool refresh when the upload is attempted.
 
-=item B<vol-download> [I<--pool> I<pool-or-uuid>] [I<--offset> I<bytes>]
-[I<--length> I<bytes>] [I<--sparse>] I<vol-name-or-key-or-path> I<local-file>
+=item B<vol-download> I<vol-name-or-key-or-path> I<local-file>
+[I<--pool> I<pool-or-uuid>] [I<--offset> I<bytes>] [I<--length> I<bytes>]
+[I<--sparse>]
 
 Download the contents of a storage volume to I<local-file>.
+
+I<vol-name-or-key-or-path> is the name or key or path of the volume to
+download into I<local-file>.
+
 I<--pool> I<pool-or-uuid> is the name or UUID of the storage pool the volume
-is in.
-I<vol-name-or-key-or-path> is the name or key or path of the volume to download.
-If I<--sparse> is specified, this command will preserve volume sparseness.
+is in. If the volume name is provided instead of the key or path, then
+providing the pool is necessary to find the volume to be uploaded into;
+otherwise, the first volume found by the key or path will be used.
+
 I<--offset> is the position in the storage volume at which to start reading
-the data. The value must be 0 or larger. I<--length> is an upper bound of
-the amount of data to be downloaded. A negative value is interpreted as
-an unsigned long long value to essentially include everything from the
-offset to the end of the volume.
+the data. The value must be 0 or larger.
+
+I<--length> is an upper bound of the amount of data to be downloaded.
+A negative value is interpreted as an unsigned long long value to
+essentially include everything from the offset to the end of the volume.
+
+If I<--sparse> is specified, this command will preserve volume sparseness.
 
 =item B<vol-wipe> [I<--pool> I<pool-or-uuid>] [I<--algorithm> I<algorithm>]
 I<vol-name-or-key-or-path>