snapshot: improve snapshot-list error message

https://bugzilla.redhat.com/show_bug.cgi?id=869100 mentioned some
confusion about 'virsh snapshot-list' errors.  Clean up a
misleading error message, and add some documentation.

* tools/virsh-snapshot.c (cmdSnapshotList): Mention --current
rather than --from when appropriate.
* tools/virsh.pod (snapshot-list): Mention that the named starting
point is NOT part of the list except under --tree.

tools/virsh-snapshot.c

@@ -1197,6 +1197,8 @@ cmdSnapshotList(vshControl *ctl, const vshCmd *cmd)
         }
         if (from) {
             vshError(ctl, "%s",
+                     vshCommandOptBool(cmd, "current") ?
+                     _("--roots and --current are mutually exclusive") :
                      _("--roots and --from are mutually exclusive"));
             goto cleanup;
         }

tools/virsh.pod

@@ -2764,7 +2764,9 @@ start at the current snapshot.  When used in isolation or with
 I<--parent>, the list is limited to direct children unless
 I<--descendants> is also present.  When used with I<--tree>, the
 use of I<--descendants> is implied.  This option is not compatible
-with I<--roots>.
+with I<--roots>.  Note that the starting point of I<--from> or
+I<--current> is not included in the list unless the I<--tree>
+option is also present.
 
 If I<--leaves> is specified, the list will be filtered to just
 snapshots that have no children.  Likewise, if I<--no-leaves> is