Use double hyphens when prefixing command options in the documentation

Use double hyphens when prefixing command options in the documentation

This makes our man pages follow the GNU long option recommandations
where non-single character options are prefixed with a double hyphen
(https://www.gnu.org/software/libc/manual/html_node/Argument-Syntax.html)

The benefit for PVE is that our documentation looks more similar to what
a user with previous Linux knowledge is used to.

Our bash autocompletion helper only completes options using double hyphens too.