docs: small overall documentation improvements (#1381)

* docs: small overall documentation improvements

- Add documentation comments to various items
- Change web API return types from bool/Option to Result
- Create helper functions with descriptive names instead of comments
- Remove redundant/confusing types
- Fix some documentation comments as instructed by `cargo doc`
- Rename variables to clear names

* docs: small fixes to the documentation update

src/traits.rs

@@ -35,6 +35,7 @@ pub trait ListItem: Sync + Send + 'static {
     }
     fn share_url(&self) -> Option<String>;
 
+    /// Get the album that contains this [ListItem].
     fn album(&self, _queue: &Queue) -> Option<Album> {
         None
     }