pgsql: Improve release note explanation of the change in libpq's

Log Message:
-----------
Improve release note explanation of the change in libpq's handling of
default usernames versus Kerberos tickets.  Per confusion about what
bug #4824 was really about.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        release-8.4.sgml (r1.6 -> r1.7)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-8.4.sgml?r1=1.6&r2=1.7)