Api-ref: add 'all_tenants' API query option

Cinder REST API allows one to use a special query parameter
named "all_tenants" to list resources from all projects (tenants)
through the REST API:

`/v3/{project_id}/volumes/detail?all_tenants=1`

However, this option is hardly documented anywhere besides
`/v3/{project_id}/volumes/summary`. So add them.

Change-Id: Ia9cc9e20c3b333a054c90f07e952b61dfad8529e
Closes-Bug: #1743800

api-ref/source/v3/os-cgsnapshots-v3.inc

@@ -82,6 +82,7 @@ Request
 .. rest_parameters:: parameters.yaml
 
    - project_id: project_id_path
+   - all_tenants: all-tenants
 
 
 Response Parameters
@@ -120,6 +121,7 @@ Request
 .. rest_parameters:: parameters.yaml
 
    - project_id: project_id_path
+   - all_tenants: all-tenants
 
 
 Response Parameters