def mark_string(string_migration, comment_format, mark):
    """Mark a string migration for proofreading.

    Adds a comment in the beginning of this string.

    :param StringMigration string_migration: the string migration object
        to update
    :param unicode comment_format: the format of the comment,
        compatible with the file type of the migration
    :param str mark: the string to add as a comment
    """
    string_migration.update('', comment_format.format(mark), append=False)
