api-ref: add 'protected' query filter

Adds documentation for the 'protected' query filter introduced
in Pike.

Change-Id: I7c63ae1715e1bd35587f0f0954596ae1263d428b
Closes-bug: #1713177

api-ref/source/v2/images-images-v2.inc

@@ -452,6 +452,7 @@ Request
    - marker: marker
    - name: name-in-query
    - owner: owner-in-query
+   - protected: protected-in-query
    - status: status-in-query
    - tag: tag-in-query
    - visibility: visibility-in-query

api-ref/source/v2/images-parameters.yaml

@@ -156,6 +156,14 @@ owner-in-query:
   in: query
   required: false
   type: string
+protected-in-query:
+  description: |
+    Filters the response by the 'protected' image property.  A valid value is
+    one of 'true', 'false' (must be all lowercase).  Any other value will
+    result in a 400 response.
+  in: query
+  required: false
+  type: boolean
 size_max:
   description: |
     Filters the response by a maximum image size, in