Adjust back-branch release note description of commits a2a718b2 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.