api-ref: fix link to flavor extra specs docs

This fixes the link, re-words it a bit, moves it to the main
description (since it applies to PUT also) and drops the note
since we don't need note formatting for linking in reference
material.

Closes-Bug: #1814953

Change-Id: Ia24cda353bdcadf3fe8405aac588e8abf1100608

api-ref/source/os-flavor-extra-specs.inc

@@ -7,6 +7,10 @@
 Lists, creates, deletes, and updates the extra-specs or keys for a
 flavor.
 
+Refer to `Compute Flavors
+<https://docs.openstack.org/nova/latest/user/flavors.html#extra-specs>`__
+for available built-in extra specs.
+
 List Extra Specs For A Flavor
 =============================
 
@@ -48,11 +52,6 @@ Create Extra Specs For A Flavor
 
 Creates extra specs for a flavor, by ID.
 
-.. note:: Please reference:
-  `Compute Flavors
-  <http://docs.openstack.org/admin-guide/compute-flavors.html#extra-specs>`__
-  for available built-in extra specs under ``Extra Specs`` section.
-
 Normal response codes: 200
 
 Error response codes: unauthorized(401), forbidden(403), itemNotFound(404),