qemu-nbd: Minor texi updates

qemu-nbd: Minor texi updates

- Change some spacing.
- Add disconnect usage to synopsis.
- Highlight the command and its options in the synopsis.
- Fix up the grammar in the description.
- Move filename variable description out of the option table.
- Add a description of the dev variable.
- Remove duplicate entry for --format.
- Reword --discard documentation.
- Add --detect-zeroes documentation.
- Add reference to qemu man page to see also section.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Message-Id: <1451979212-25479-3-git-send-email-sitsofe@yahoo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>