Document+check commitment_signed generation success on send_htlc

Document+check commitment_signed generation success on send_htlc

Because we don't have an HTLCState for
update_add_htlc-generated-but-not-yet-commitment_signed to simplify
the mess of HTLCState match arms, any time a Channel::send_htlc
call returns Ok(Some(_)) we MUST call commitment_signed and it MUST
return success (or close the channel). We mention this in the docs
and panic if its not met in ChannelManager (which lets the fuzz
tester check this).