LSPS1: Add missing docs

[tnull]

In #58 we added most of the docs missing from LSPS1.

However, we also left some TODOs, in particular in lsps1/event.rs. Moreover, we for now hid all pub fn from LSPS1MessageHandler for them to be exposed-and-documented as needed at a later date.

We should see to add the missing docs before removing the cfg guard.