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@gmail.com>
Discussion: https://postgr.es/m/CAAAe_zAFz1v-3b7Je4L%2B%3DwZM3UGAczXV47YVZfZi9wbJxspxeA%40mail.gmail.com