logind: slightly tweak error message about not enough swap for hibernation

logind: slightly tweak error message about not enough swap for hibernation

Let's tweak the message if not enough swap is around slightly: systems
might have plenty swap backed by incompatible storage (specifically:
swap files on btrfs), but we (currently) do not support hibernating to
that.

Hence let's say *suitable* swap space and talk about *compatibility* of
backing storage.

Hopefully this will make things a bit clearer to users.

Prompted by: #29189