pgsql: Fix pg_wchar_table[] to match revised ordering of the encoding ID

Log Message:
-----------
Fix pg_wchar_table[] to match revised ordering of the encoding ID enum.
Add some comments so hopefully the next poor sod doesn't fall into the
same trap.  (Wrong comments are worse than none at all...)

Modified Files:
--------------
    pgsql/src/backend/utils/mb:
        encnames.c (r1.35 -> r1.36)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/encnames.c?r1=1.35&r2=1.36)
        wchar.c (r1.64 -> r1.65)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/wchar.c?r1=1.64&r2=1.65)
    pgsql/src/include/mb:
        pg_wchar.h (r1.74 -> r1.75)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/mb/pg_wchar.h?r1=1.74&r2=1.75)