Improved search and its docs

[allerter]

• _get_item_from_search_response includes all items in the hits list as long as their type matches. But this can lead to false matches. For example, the user could be searching for a song. The method would include songs that have similarities in their lyrics to the search term. But what we actually want is songs that have a similar title. So now the method takes the index key into account as well.
• Made clear in search_album, search_artist and search_song methods that they're only convenience methods and the user can do what the method does themselves.
• Improved the meaning of the type_id (e.g. song_id`) parameter in the convenience methods.