Further clarify `register_tx`/`register_output` distinction in docs

[TheBlueMatt]

See getlipa/ldk-sample-java#1 (comment) for a concrete suggestion, but it seems there continues to be some confusion from users over the distinction here.

[johncantrell97]

For me the confusion wasn't around the distinction between register_tx and register_output. By providing both the txid and script_pubkey to register_tx made me feel like I might be misunderstanding something about how I needed to be watching for the transaction.

For example, when using BIP157/158 you are able to watch for this transaction by filtering on the script alone. The txid is unnecessary. It might help me add to do the docs something similar to:

'While we provide both the txid and the script_pubkey, depending on how you manage your chain data, you might only use one or both of these pieces of information to monitor for the transaction".

[gabbyprecious]

Hi @ConorOkus is this available to work on?

[ConorOkus]

Yes sure @gabbyprecious

[tnull]

Will pick this up.

[tnull]

Will pick this up.

Ah, I just saw now that @gabbyprecious indicated in #1395 that she still intends to work on this. Holding off for now.