scripts/qemu-guest-agent/fsfreeze-hook: improve script description

scripts/qemu-guest-agent/fsfreeze-hook: improve script description

With the current wording, users might think that the -F option is not
required as long as the script is placed in the default path. Be clear
that the option is always required. Also includes some minor language
improvements in the rest of the comment.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Kostiantyn Kostiuk <kkostiuk@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250916122111.36019-1-f.ebner@proxmox.com
Signed-off-by: Kostiantyn Kostiuk <kkostiuk@redhat.com>