Default coin selection used by `TxBuilder` is not documented

[thunderbiscuit]

I think we should make the default coin selection algo more explicit in our docs if we can. Just ran into this while reviewing a bindings PR and had to dig into it to figure it out.

[Arowolokehinde]

Hi @thunderbiscuit this would be my first contribution to bdk_wallet.
Would it be alright if I work on this ? I understand the ask is to document the default coin selection algorithm used by TxBuilder.

I noticed there's an open PR referencing this issue . let me know if that's still active or if this is open for a fresh attempt. Happy to submit a PR if you give me a go ahead.