pgsql: Fix comments for Korean encodings in encnames.c

Fix comments for Korean encodings in encnames.c

* JOHAB: replace the incorrect "simplified Chinese" description with
a correct one that identifies it as the Korean combining (Johab)
encoding standardized in KS X 1001 annex 3.

* EUC_KR: drop a stray space before the comma in the existing
comment, and note that the encoding covers the KS X 1001
precomposed (Wansung) form.

* UHC: spell out "Unified Hangul Code", clarify that it is
Microsoft Windows CodePage 949, and describe its relationship to
EUC-KR (superset covering all 11,172 precomposed Hangul syllables).

Backpatch-through: 14
Author: Henson Choi <assam258(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAAAe_zAFz1v-3b7Je4L%2B%3DwZM3UGAczXV47YVZfZi9wbJxspxeA%40mail.gmail.com

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e5fb1ff5bed9778f159a658ff098db97c8ec8536

Modified Files
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src/common/encnames.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)