pgsql: doc: Document pg_encoding_to_char() and pg_char_to_encoding().

doc: Document pg_encoding_to_char() and pg_char_to_encoding().

Previously both functions were not described anywhere in the docs.
But since they have been around since 7.0 and mentioned in the description
for system catalog like pg_database, it's reasonable to add short
descriptions for them.

Author: Ian Lawrence Barwick
Reviewed-by: Laurenz Albe, Fujii Masao
Discussion: https://postgr.es/m/CAB8KJ=infievn4q1N4X7Vx8w4_RMPPG0pLvxhSDjy5WQOSHW9g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f6b5d05ba9a4ac7c5ebec76045c6e0afcf7c9eec

Modified Files
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doc/src/sgml/catalogs.sgml |  8 +++++---
doc/src/sgml/func.sgml     | 30 ++++++++++++++++++++++++++++++
2 files changed, 35 insertions(+), 3 deletions(-)