pgsql: doc: add xreflabel text for libpq and PL/Python

doc: add xreflabel text for libpq and PL/Python

to be used for PG 18 release notes

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d8aa21b74ff4e3d767c3344484c3cb22b9f0ec0d

Modified Files
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doc/src/sgml/libpq.sgml | 2 +-
doc/src/sgml/plpython.sgml | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)