guest-agent: document allow-rpcs in config file section

guest-agent: document allow-rpcs in config file section

While the `allow-rpcs` option is documented in the CLI options
section, it was missing in the section about the configuration file
syntax.

And while it's mentioned that "the list of keys follows the command line
options", having `block-rpcs` there but not `allow-rpcs` seems like
being a potential source of confusion; and as it's cheap to add let's
just do so.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Message-ID: <20240718140407.444160-1-t.lamprecht@proxmox.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>