This adds documentation for both the short and long form of many
options (which was inconsistent before: in some cases only the short
form was used, in others only the long form).

Note that the standard this patch adopts is to document both forms as
such:

  {-O|--os-parameters} …

This makes it a bit uglier in complex situations, but the alternatives
considered were not perfect either. Other suggestions (with patches)
welcome.

Additionally, it fixes two doc bugs:

- in gnt-cluster.rst, the --prealloc-wipe-disks section was in the
  middle of a paragraph
- in gnt-instance.rst, a list was not typed correctly, thus it was
  mangled as a single paragraph

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>