pgsql: Some documentation cleanup for the addition of the KOI8U

Log Message:
-----------
Some documentation cleanup for the addition of the KOI8U encoding.  Change
all (remaining) mentions of KOI8 to the new canonical form KOI8R.  Add
information about the available conversions for KOI8U.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        charset.sgml (r2.94 -> r2.95)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/charset.sgml?r1=2.94&r2=2.95)
        func.sgml (r1.479 -> r1.480)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.479&r2=1.480)