Add more documentation to collab (#4095)

This PR adds more documentation to the `collab` crate.

Release Notes:

- N/A

---------

Co-authored-by: Conrad <conrad@zed.dev>

crates/collab/src/db/tables/user.rs

@@ -2,6 +2,7 @@ use crate::db::UserId;
 use sea_orm::entity::prelude::*;
 use serde::Serialize;
 
+/// A user model.
 #[derive(Clone, Debug, Default, PartialEq, Eq, DeriveEntityModel, Serialize)]
 #[sea_orm(table_name = "users")]
 pub struct Model {