Add disclaimer to broadcast/chain endpoints in docs

[kilrau]

We want to add a disclaimer that using

https://api.boltz.exchange/swagger#/Chain/get_chain__currency__transaction__id_ https://api.boltz.exchange/swagger#/Chain/post_chain__currency__transaction

involves trust in Boltz and that there are self-hosted and 3rd party solutions. We can only really push folks to do that once cheapCT on Liquid is available as our endpoints are needed for lowball to function.