[api-ref] Add note about no validation of the target_tenant in RBAC API

Neutron can't really validate if string provided as 'target_tenant' in
the /v2.0/rbac-policies POST/PUT request is project ID or something
else, like e.g. project's name.
This patch adds small note about this to the Neutron api-ref document so
that users can be more aware of that.

Related-Bug: #2065743
Change-Id: Ie5bec404c372209ca15e4c8454957040032751cb

api-ref/source/v2/parameters.yaml

@@ -7237,6 +7237,16 @@ target_tenant:
   in: body
   required: true
   type: string
+target_tenant-rbac-create-update:
+  description: |
+    The ID of the tenant to which the RBAC policy will be enforced. Please note
+    that Neutron does not perform any type of validation that the value provided is
+    actually the ID of the existing project. If, for example, the name of the project
+    is provided here, it will be accepted by the Neutron API, but the RBAC rule
+    created will not work as expected.
+  in: body
+  required: true
+  type: string
 tas_id:
   description: |
     Tha id for the Tap Service.

api-ref/source/v2/rbac-policy.inc

@@ -77,7 +77,7 @@ Request
 .. rest_parameters:: parameters.yaml
 
    - rbac_policy_id: rbac_policy_id-path
-   - target_tenant: target_tenant
+   - target_tenant: target_tenant-rbac-create-update
 
 Request Example
 ---------------
@@ -191,7 +191,7 @@ Request
 
 .. rest_parameters:: parameters.yaml
 
-   - target_tenant: target_tenant
+   - target_tenant: target_tenant-rbac-create-update
    - object_type: object_type
    - object_id: object_id
    - action: rbac_action