pgsql: Adjust back-branch release note description of commits a2a718b22

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.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/aa078a9f45099e6a385445a4350ed2af94154070

Modified Files
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doc/src/sgml/release-9.0.sgml |    4 ++--
doc/src/sgml/release-9.1.sgml |    4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)