pgsql: doc Makefile: issue warning about chars that cannot be output

doc Makefile: issue warning about chars that cannot be output

A follow-up improvement to commit 641a5b7a144.

Reported-by: Tatsuo Ishii, Tom Lane

Discussion: https://postgr.es/m/20241126.182513.1752581942460106099.ishii@postgresql.org

Backpatch-through: master

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e4c8865196f6ad6bb3473bcad1d2ad51147e4513

Modified Files
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doc/src/sgml/Makefile | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)