docs: document year and year_month display_type host contract

scripts/db/display_metadata.py

@@ -22,6 +22,8 @@ LEGACY_METADATA_TABLES = frozenset({"_schema_meta"})
 METADATA_TABLES = INTERNAL_METADATA_TABLES | LEGACY_METADATA_TABLES
 
 DATETIME_UNIX_SECONDS = "datetime_unix_seconds"
+YEAR_MONTH = "year_month"
+YEAR = "year"
 STANDARD_TIMESTAMP_COLUMNS = frozenset({"created_at", "updated_at"})
 
 _JIANGCHANG_TABLES_DDL = """