Document support for "in" operator in query filters

As of OPENIDM-14362 and this PR this PR, query filters now support the "IN" expression operator, which is used when querying for a value in an array of objects. For example: http://idm.example.com:8080/openidm/managed/user?_pageSize=1000&_fields=*&_queryFilter=/userName+IN+'["user3a","user4a"]'

We cover the supported query expressions here, so it should be added there and also in the appendix.