Fix documentation for contains, does_not_contain, contains_case_insensitive, does_not_contain_case_insensitive

[gerrycampion]

The documentation mentions substrings, but these operators do not use substring matching. Refer to CG0067 for an example.

https://github.com/cdisc-org/business-rules/blob/710faeef42bb1e1bbf8c3eefae17e513c98a9115/business_rules/operators.py#L536