FHIR-51043 - add note on _since to _typeFIlter docs (#151)

Author: gotdan

input/pagecontent/export.md

@@ -288,6 +288,8 @@ To obtain new and updated resources for patients in a group, as well as all data
 
 The `_typeFilter` parameter enables finer-grained filtering out of resources in the bulk data export response that would have otherwise been returned. For example, a client may want to retrieve only active prescriptions rather than all prescriptions and only laboratory observations rather than all observations. When using `_typeFilter`, each resource type is filtered independently. For example, filtering `Patient` resources to people born after the year 2000 will not filter `Encounter` resources for patients born before the year 2000 from the export.
 
+Filtering resources based on the dates associated with a clinical or administrative event, such as exporting encounters that occurred within a certain time period, should be done using the `_typeFilter` parameter and not the `_since` and `_until` parameters, since the resource modification date used in those filters may not correspond to the date of the clinical or administrative event.
+
 The value of the `_typeFilter` parameter is a FHIR REST API query. Resources with a resource type specified in this query that do not meet the criteria in the search expression in the query SHALL NOT be returned, with the exception of related resources being included by a server to provide context about the resources being exported (see [processing model](#processing-model)). A client MAY repeat the `_typeFilter` parameter multiple times in a kick-off request. When more than one `_typeFilter` parameter is provided with a query for the same resource type, the server SHALL include resources of that resource type that meet the criteria in any of the parameters (a logical "or").  
 
 FHIR [search result parameters](https://www.hl7.org/fhir/search.html#modifyingresults) (such as _sort, _include, and _elements) SHALL NOT be used as `_typeFilter` criteria. Additionally, a query in the `_typeFilter` parameter SHALL have the [search context](https://hl7.org/fhir/search.html#searchcontexts) of a single FHIR Resource Type. The contexts "all resource types" and "a specified compartment" are not allowed. Clients should consult the server's capability statement to identify supported search parameters (see [server capability documentation](#server-capability-documentation)). Since support for `_typeFilter` is OPTIONAL for a FHIR server, clients SHOULD be robust to servers that ignore `_typeFilter`.