Update description of valid whitelist for non-admin user

Non-admin user can filter instance by instance-uuid and other
filter keys with being configured using the "os_compute_api:
servers:allow_all_filters" policy rule. The policy rule was
added with Ia5504da9a00bad689766aeda20255e10b7629f63.

Closes-Bug:1819425

Change-Id: I74146b4370b6017cb199466258165fc6df080082

Author: 翟小君

api-ref/source/servers.inc

@@ -154,7 +154,9 @@ There is whitelist for valid filter keys. Any filter key other than from
 whitelist will be silently ignored.
 
 - For non-admin users, whitelist is different from admin users whitelist.
-  Valid whitelist for non-admin users includes
+  The valid whitelist can be configured using the
+  ``os_compute_api:servers:allow_all_filters`` policy rule. By default,
+  the valid whitelist for non-admin users includes
 
   - ``all_tenants``
   - ``changes-since``