Cleanup docs for DNS lookup configure options

DNS lookups for KDCs have been unconditionally enabled in
configure since 2003; configure options only affect whether
DNS lookups are used for realm names.
Change the RST documentation of configure options to catch up.

doc/rst_source/krb_build/options2configure.rst

@@ -46,13 +46,12 @@ Most commonly used options
     library (TCLPATH/lib) should be found.
 
 **--enable-dns-for-realm**
-    Enable the use of DNS to look up a host's Kerberos realm, or a
-    realm's KDCs, if the information is not provided in
-    :ref:`krb5.conf(5)`.  See :ref:`kdc_hostnames` for information
-    about using DNS to locate the KDCs, and :ref:`mapping_hostnames`
+    Enable the use of DNS to look up a host's Kerberos realm,
+    if the information is not provided in
+    :ref:`krb5.conf(5)`.  See
+    :ref:`mapping_hostnames`
     for information about using DNS to determine the default realm.
-    By default, DNS lookups are enabled for the former but not for the
-    latter.
+    DNS lookups for realm names are disabled by default.
 
 **--with-system-et**
     Use an installed version of the error-table (et) support software,