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/commands.rs

@@ -238,8 +238,8 @@ impl CommandManager {
             }
             Command::NewPlaylist(name) => {
                 match self.spotify.api.create_playlist(name, None, None) {
-                    Some(_) => self.library.update_library(),
-                    None => error!("could not create playlist {}", name),
+                    Ok(_) => self.library.update_library(),
+                    Err(_) => error!("could not create playlist {}", name),
                 }
                 Ok(None)
             }