Add guidance on "POST ... _search" in general guidance here: http://build.fhir.org/ig/HL7/US-Core/general-guidance.html#search-syntax

and reference the relevant section in the base specification that describes the server expectations.

here: 

http://build.fhir.org/search.html#Introduction

and here:

http://build.fhir.org/http.html#search