[placement] Add 'CUSTOM_' prefix description in API ref

In the following resource class APIs,
the name of a resource class must start with 'CUSTOM_'.
If not, the request returns a 'Bad Request (400)' response code.
It should be described in the API reference. So add it.

* POST /resource_classes
* PUT /resource_classes/{name}

Change-Id: I132c532678bb74a460515067187fbf1e30885335
Closes-Bug: #1733308

placement-api-ref/source/parameters.yaml

@@ -11,6 +11,14 @@ resource_class_path: &resource_class_path
   required: true
   description: >
     The name of one resource class.
+resource_class_path_custom: &resource_class_path_custom
+  type: string
+  in: path
+  required: true
+  description: >
+    The name of one resource class. The name must start with
+    the prefix ``CUSTOM_``. If not, the request returns a ``Bad Request (400)``
+    response code.
 resource_provider_uuid_path: &resource_provider_uuid_path
   type: string
   in: path
@@ -200,6 +208,9 @@ reserved_opt:
 resource_class:
   <<: *resource_class_path
   in: body
+resource_class_custom:
+  <<: *resource_class_path_custom
+  in: body
 resource_class_links:
   type: array
   in: body

placement-api-ref/source/resource_class.inc

@@ -64,7 +64,7 @@ Request
 
 .. rest_parameters:: parameters.yaml
 
-  - name: resource_class_path
+  - name: resource_class_path_custom
 
 Response
 --------
@@ -92,7 +92,8 @@ Request
 
 .. rest_parameters:: parameters.yaml
 
-  - name: resource_class
+  - name: resource_class_path
+  - name: resource_class_custom
 
 Request example
 ---------------

placement-api-ref/source/resource_classes.inc

@@ -51,7 +51,7 @@ Request
 
 .. rest_parameters:: parameters.yaml
 
-  - name: resource_class
+  - name: resource_class_custom
 
 Request example
 ---------------