virsh: undocument --shareable (--mode already covers it)

virsh: undocument --shareable (--mode already covers it)

Commit e962a57 added 'attach-disk --shareable', even though we
already had 'attach-disk --mode=shareable'.  Worse, if the user
types 'attach-disk --mode=readonly --shareable', we create
non-sensical XML.  The best solution is just to undocument the
duplicate spelling, by having it fall back to the preferred
spelling.

* tools/virsh-domain.c (cmdAttachDisk): Let alias handling fix our
mistake in exposing a second spelling for an existing option.
* tools/virsh.pod: Fix documentation.

Signed-off-by: Eric Blake <eblake@redhat.com>