Doc: fix character_sets view.

Doc: fix character_sets view.

The note regarding character encoding form in "The Information Schema"
said that LATIN1 character repertoires only use one encoding form
LATIN1. This is not correct because LATIN1 has another encoding form
ISO-2022-JP-2. To fix this, replace LATIN1 with LATIN2, which is not
supported by ISO-2022-JP-2, thus it can be said that LATIN2 only uses
one encoding form.

Back-patch to supported branches.

Author: Tatsuo Ishii
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/flat/20240102.153925.1147403616414525145.t-ishii%40sranhm.sra.co.jp