> To list backups that will should be kept use the --invert option.

May I suggest changing the name to something more direct? Calling it "--invert" means the user must think through what the default sense of the test is, then negate that in their mind. Not exactly a tough mental task, but people do make careless errors.

Perhaps something like "--list=keep" and "--list=discard" (with the default being "discard").

Also, typos:

"will make prunef to keep" --> "will make prunef keep"

"list backups that will should be" --> "list backups that should be"