Update doc comment for prefixTableName

Signed-off-by: S1m <31284753+p1gp1g@users.noreply.github.com>

lib/private/DB/QueryBuilder/QueryBuilder.php

@@ -1339,6 +1339,8 @@ class QueryBuilder implements IQueryBuilder {
 	/**
 	 * Returns the table name with database prefix as needed by the implementation
 	 *
+	 * Was protected until version 30.
+  	 *
 	 * @param string $table
 	 * @return string
 	 */