Updated flavors doc

Flavor with special characters in flavor ID is not allowed to create

Closes-Bug: 1920938
Change-Id: I600ae07f7fb039c9ccf461c4bf74ae42d9a3744f

api-ref/source/parameters.yaml

@@ -3129,8 +3129,9 @@ flavor_id_body_2_46:
   max_version: 2.46
 flavor_id_body_create:
   description: |
-    The ID of the flavor. While people often make this look like an int, this
-    is really a string. If not provided, this defaults to a uuid.
+    Only alphanumeric characters with hyphen '-', underscore '_', spaces
+    and dots '.' are permitted. If an ID is not provided, then a default UUID
+    will be assigned.
   in: body
   required: false
   type: string