docs: fix minor things in man-page howto

Signed-off-by: Karel Zak <kzak@redhat.com>
fdisk: add long options, cleanup man page and usage()

Signed-off-by: Karel Zak <kzak@redhat.com>
bash-completion: update fdisk

Signed-off-by: Karel Zak <kzak@redhat.com>