Fix some release notes in preparation for the o-2 beta release

There were several release notes which are not yet released
but will be in the o-2 beta so this goes through and cleans up
some typos, broken links and grammar issues.

Change-Id: Ic5bcf43e94e09c59b2e16807c55d84046d90c96f

releasenotes/notes/bug-1636157-2148ea3675969a5d.yaml

@@ -5,6 +5,6 @@ features:
      was being used for all actions (show, index, delete, create)
      for server_groups REST APIs. It was thus impossible to provide
      different RBAC for specific actions based on roles. To address
-     this changes are made to have separate policy rules for each
+     this changes were made to have separate policy rules for each
      action. The original rule (os_compute_api:os-server-groups) is
      left unchanged for backward compatibility.

releasenotes/notes/placement-rest-custom-resource-classes-a3f2175772983b0a.yaml

@@ -7,4 +7,4 @@ features:
     resources that are not interoperable between OpenStack clouds. See the
     `Placement REST API Version History`_ documentation for usage details.
 
-    .. _Placement REST API Version History: http://docs.openstack.org/developer/nova/placement.html#id3
+    .. _Placement REST API Version History: http://docs.openstack.org/developer/nova/placement.html#id2

releasenotes/notes/remove-deprecated-compute-options-dbf2be75d6bdbcc8.yaml

@@ -1,5 +1,6 @@
 ---
 upgrade:
   - |
-    The ``compute_stats_class`` options is deprecated and have been removed
-    from the ``default`` group of nova.conf.
+    The ``compute_stats_class`` configuration option was deprecated since the
+    13.0.0 Mitaka release and has been removed. Compute statistics are now always
+    generated from the ``nova.compute.stats.Stats`` class within Nova.

releasenotes/notes/remove-scheduler_json_config_location-option-c669e8c9867ce0fb.yaml

@@ -2,4 +2,4 @@
 upgrade:
   - |
     The ``scheduler_json_config_location`` configuration option has not been
-    used in recent releases and has been removed.
+    used since the 13.0.0 Mitaka release and has been removed.

releasenotes/notes/remove_deprecated_cert_topic_conf_opt-6402aeca8629da95.yaml

@@ -1,5 +1,4 @@
 ---
 upgrade:
   - |
-    ``cert_topic`` has been removed from the ``DEFAULT`` section. It was marked
-    as deprecated due to planned removal of nova-cert service.
+    The deprecated ``cert_topic`` configuration option has been removed.

releasenotes/notes/rm-deprecated-compute_available_monitors-option-c8d0b81304452786.yaml

@@ -2,5 +2,5 @@
 upgrade:
   - |
     The deprecated config option ``compute_available_monitors`` has been removed
-    from the ``Default`` config section. Use setuptools entry points to list
+    from the ``DEFAULT`` config section. Use setuptools entry points to list
     available monitor plugins.