pgsql: doc: Fix descriptions related to the handling of non-ASCII chara

doc: Fix descriptions related to the handling of non-ASCII characters

Since 45b1a67a0fcb, non-printable ASCII characters do not show up in
various configuration paths as question marks, but as hexadecimal
escapes.  The documentation was not updated to reflect that.

Author: Hayato Kuroda
Reviewed-by: Jian He, Tom Lane, Karl O. Pinc, Peter Smith
Discussion: https://postgr.es/m/TYAPR01MB586631D0961BF9C44893FAB1F523A@TYAPR01MB5866.jpnprd01.prod.outlook.com
Backpatch-through: 16

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/950e236e50691d80e50d0aa18b66d1f459105558

Modified Files
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doc/src/sgml/config.sgml       | 15 +++++++++------
doc/src/sgml/postgres-fdw.sgml |  6 +++---
2 files changed, 12 insertions(+), 9 deletions(-)