[api-ref] add provider_location to snapshots

This optional field was missing in the docs

Change-Id: I86a3161b098652788f8a982053a8efd06a73dbba

api-ref/source/parameters.yaml

@@ -3406,6 +3406,16 @@ snapshot_provider_location:
   in: body
   required: true
   type: string
+snapshot_provider_location_optional:
+  description: |
+    Provider location of the snapshot on the backend. This parameter is only
+    available to users with an "administrator" role by virtue of default
+    RBAC. This behavior can be modified by overriding the
+    ``context_is_admin`` policy via a custom ``policy.yaml``.
+  in: body
+  required: false
+  type: string
+  min_version: 2.12
 snapshot_provider_location_request:
   description: |
     Provider location of the snapshot on the backend.

api-ref/source/snapshots.inc

@@ -133,6 +133,7 @@ Response parameters
    - size: snapshot_size
    - project_id: snapshot_project_id
    - user_id: snapshot_user_id
+   - provider_location: snapshot_provider_location_optional
 
 Response example
 ----------------
@@ -185,6 +186,7 @@ Response parameters
    - size: snapshot_size
    - project_id: snapshot_project_id
    - user_id: snapshot_user_id
+   - provider_location: snapshot_provider_location_optional
 
 Response example
 ----------------
@@ -248,7 +250,7 @@ Response parameters
    - created_at: created_at
    - share_proto: snapshot_share_protocol
    - share_size: snapshot_share_size
-   - provider_location: snapshot_provider_location
+   - provider_location: snapshot_provider_location_optional
    - size: snapshot_size
    - project_id: snapshot_project_id
    - user_id: snapshot_user_id