Clarify "Cover unifiers" rule

Main point is to adjust the explanation to allow usage of existing artist entries (was not clear before asking for clarification on Discord).

Other than that, clearer language and added excerpt

Author: Hans5958

src/content/rules/cover-unifiers.mdx

@@ -19,8 +19,9 @@ mikumod_support: "True"
 relevant_tag_id: 6751
 automatically_fixed: "False"
 complete_validation: "True"
+excerpt: You may create cover unifier song entries for out-of-scope songs if it has 5 derived song entries on VocaDB. If the artist doesn't exist on VocaDB, add it as a custom artist.
 ---
 
-If the out-of-scope original version has 5 or more derived song entries (such as remixes or covers) on VocaDB, it can be created as a [cover unifier](https://vocadb.net/T/6751) song entry.
+If an out-of-scope song has at least 5 song entries as alternate versions (such as remixes or covers) on VocaDB, it can be created as a [cover unifier](https://vocadb.net/T/6751) song entry.
 
-Cover unifier artists who are out-of-scope should be credited as "custom artists" (= no entry). The [Temporary artist entry](https://vocadb.net/Ar/139625) can be used to bypass the creation form.
+Do not create artist entries solely for these cover unifiers. You can add existing artist entries to a song entry if it is added as an artist on an in-scope entry (e.g. using "[HACHI](https://vocadb.net/Ar/49)" for "[KICK BACK](https://vocadb.net/S/454815)"), but do not create new artist entries just for crediting a cover unifier. When no artist entry can be added, you can use [the "Temporary artist" entry](https://vocadb.net/Ar/139625) so that you can create the song entry.