Drop return value from `fail_htlc_backwards`, clarify docs

Drop return value from `fail_htlc_backwards`, clarify docs

`ChannelManager::fail_htlc_backwards`' bool return value is quite
confusing - just because it returns false doesn't mean the payment
wasn't (already) failed. Worse, in some race cases around shutdown
where a payment was claimed before an unclean shutdown and then
retried on startup, `fail_htlc_backwards` could return true even
though (a duplicate copy of the same payment) was claimed, but the
claim event has not been seen by the user yet.

While its possible to use it correctly, its somewhat confusing to
have a return value at all, and definitely lends itself to misuse.

Instead, we should push users towards a model where they don't care
if `fail_htlc_backwards` succeeds - either they've locally marked
the payment as failed (prior to seeing any `PaymentReceived`
events) and will fail any attempts to pay it, or they have not and
the payment is still receivable until its timeout time is reached.

We can revisit this decision based on user feedback, but will need
to very carefully document the potential failure modes here if we
do.