Internal helper function for checking specific arguments used in custom_search.
check_args_string(string, section_or_field, negate)A character string specifying the word or expression to search for in messages.
A mandatory character string specifying in which messages's Section or Header Field to search for the provided string. For some available options, see section_or_field_options.
If TRUE, negates the search and seeks for
"NOT search_criterion". Default is FALSE.
NULL if arguments are correct.
Other customsearch checkargs functions: check_args_date,
check_args_flag,
check_args_size,
check_args_within