Adjust back-branch release note description of commits a2a718b22 et al.

As pointed out by Michael Paquier, recovery_min_apply_delay didn't exist
in 9.0-9.3, making the release note text not very useful.  Instead make it
talk about recovery_target_xid, which did exist then.

9.0 is already out of support, but we can fix the text in the newer
branches' copies of its release notes.

doc/src/sgml/release-9.0.sgml

@@ -1496,8 +1496,8 @@
 
      <para>
       The most notable oversight was
-      that <varname>recovery_min_apply_delay</> failed to delay application
-      of a two-phase commit.
+      that <varname>recovery_target_xid</> could not be used to stop at
+      a two-phase commit.
      </para>
     </listitem>
 

doc/src/sgml/release-9.1.sgml

@@ -1666,8 +1666,8 @@ Branch: REL9_0_STABLE [9d6af7367] 2015-08-15 11:02:34 -0400
 
      <para>
       The most notable oversight was
-      that <varname>recovery_min_apply_delay</> failed to delay application
-      of a two-phase commit.
+      that <varname>recovery_target_xid</> could not be used to stop at
+      a two-phase commit.
      </para>
     </listitem>