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>

scripts/qemu-guest-agent/fsfreeze-hook

@@ -1,11 +1,12 @@
 #!/bin/sh
 
-# This script is executed when a guest agent receives fsfreeze-freeze and
-# fsfreeze-thaw command, if it is specified in --fsfreeze-hook (-F)
-# option of qemu-ga or placed in default path (/etc/qemu/fsfreeze-hook).
-# When the agent receives fsfreeze-freeze request, this script is issued with
-# "freeze" argument before the filesystem is frozen. And for fsfreeze-thaw
-# request, it is issued with "thaw" argument after filesystem is thawed.
+# This script is executed when the guest agent receives fsfreeze-freeze and
+# fsfreeze-thaw commands, provided that the --fsfreeze-hook (-F) option of
+# qemu-ga is specified and the script is placed in /etc/qemu/fsfreeze-hook or in
+# the path specified together with -F. When the agent receives fsfreeze-freeze
+# requests, this script is called with "freeze" as its argument before the
+# filesystem is frozen. And for fsfreeze-thaw requests, it is called with "thaw"
+# as its argument after the filesystem is thawed.
 
 LOGFILE=/var/log/qga-fsfreeze-hook.log
 FSFREEZE_D=$(dirname -- "$0")/fsfreeze-hook.d