[apdavison]

Currently, a filter on a text property can be given as an Equals (the KG's EQUALS operator) or a Regex (REGEX), but the default, CONTAINS, has no class of its own: it is what a plain string does. The documentation therefore describes Equals and Regex each in terms of "a plain string", rather than presenting the three ways of matching text together.

With this PR, there is a Contains class alongside Equals and Regex (exported from fairgraph), and a plain string given as the filter value for a text property is converted to a Contains, so every text filter is explicit internally. The existence query uses Contains with existence_match="contains", as it already uses Equals with "equals". Values for other properties (ids of linked nodes, IRIs, dates, numbers) are left as they are.

A plain string means the same as before, so nothing changes for existing code: the golden query tests pass unchanged. The user guide now introduces the three filter classes together.