[docs]: Update custom note docs

[critesjosh]

feedback from a community member. These are some questions they had that they could not find answers to.

explain what does #[aztec(note)] do. Explain each field. Why do we need randomness, why do we need npk_m_hash, what does serialize_content, explain context.request_nsk_app, explain get_nsk_app(self.npk_m_hash). What if there is no npk_m_hash(in case of a multisig, i believe)? Why is trait Eq is implemented manually instead of #[derive(Eq)]?

[catmcgee]

small changes for now but this + general notes docs will be fixed in this issue #346