RUST-DB: Fix down.sql script + add comment for added languages

src/rust/liblektor-rs/migrations/2022-09-30-204512_initial/down.sql

@@ -6,4 +6,4 @@ DROP TABLE kara_tag;
 DROP TABLE tag;
 DROP TABLE kara_tags;
 DROP TABLE history;
-DROP TABLE ISO_639_1;
+DROP TABLE iso_639_1;
\ No newline at end of file

src/rust/liblektor-rs/migrations/2022-09-30-204512_initial/up.sql

@@ -95,6 +95,7 @@ INSERT OR REPLACE INTO iso_639_1 (is_macro, is_iso, code, name_en) VALUES
   ( false, true, "vi", "Vietnamese"     ),
   ( false, true, "zh", "Chinese"        ),
   ( false, true, "zu", "Zulu"           ),
-  -- ...
+  -- The things we added because they were not defined in the ISO 639-1 and
+  -- because we need them to distinguish some karas.
   ( true,  false, "fiction", "Fictional" ),
   ( true,  false, "autre",   "Joker"     );