Add docstrings for undocumented components

[spacebear21]

Start on docs.rs/payjoin to see which modules, structs, functions, enum variants... are missing documentation.

Helpful resource: https://doc.rust-lang.org/rustdoc/how-to-write-documentation.html

[spacebear21]

Something I noticed is that receive::v2 docstrings are lacking compared to receive::v1. This is unfortunate because we want to encourage v2 usage, but currently users are left in the dark (despite relevant docs existing in the inner components, but not reflected on the wrapper). Example: https://docs.rs/payjoin/latest/payjoin/receive/v2/struct.ProvisionalProposal.html#implementations

Most structs and methods are the same so the docstrings should really be mirrored between v1 and v2 components. It would be cool if there was a way to forward dosctrings declarations from the inner to the wrapper, but AFAIK that's not possible so we'll need to duplicate those entries.