update language on catalog queryables

Author: philvarner

README.md

@@ -242,8 +242,11 @@ not compliant with this extension.
 ## Queryables
 
 The Queryables mechanism allows a client to discover what terms are available for use when writing filter
-expressions.  These terms can be defined per-collection, and the intersection of these terms over all collections is what
-is available for filtering when there are no collection restrictions. By default, these queryables are the only terms that may be used
+expressions. These terms are defined both over the entire catalog (at `/queryables`) and per collection (at `/collections/{collectionId}/queryables`). The decision as to which queryables to define
+for the entire catalog is at the discretion of the implementer, and can be anywhere between none and the union of all
+queryables across all collections.
+
+By default, the queryables are the only terms that may be used
 in filter expressions, and if any term is used in expression that is not defined as a queryable and error must be
 returned according to OAFeat Part 3. It is recognized that this is a severe restriction in STAC APIs that have highly variable
 and dynamic content, so this behavior may be modified by setting the `additionalProperties` attribute in the

openapi.yaml

@@ -36,11 +36,9 @@ paths:
       description: |-
         This endpoint returns a list of variable terms that can be used in CQL2 expressions. The
         precise definition of this can be found in the OGC API - Features - Part 3: Filtering and the 
-        Common Query Language (CQL) specification.
+        Common Query Language (CQL2) specification.
       tags:
         - Filter Extension
-      # parameters:
-      # todo: may have collections parameter in the future
       responses:
         '200':
           $ref: '#/components/responses/Queryables'