pgsql: Minor GUC cleanups: document krb_server_hostname and

Log Message:
-----------
Minor GUC cleanups: document krb_server_hostname and custom_variable_classes
in postgresql.conf.sample, mark custom_variable_classes as SIGHUP not
POSTMASTER to agree with the documentation (I can't see a reason it has
to be POSTMASTER so I think the docs are right).

Modified Files:
--------------
    pgsql/doc/src/sgml:
        runtime.sgml (r1.345 -> r1.346)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.345&r2=1.346)
    pgsql/src/backend/utils/misc:
        guc.c (r1.284 -> r1.285)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.284&r2=1.285)
        postgresql.conf.sample (r1.159 -> r1.160)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.159&r2=1.160)