doc: add note about resolver as a parent

doc/man/knot.conf.5in

@@ -538,6 +538,13 @@ have a corresponding DS for the rollover to continue. If none is specified, the
 rollover must be pushed forward manually.
 .sp
 \fIDefault:\fP not set
+.sp
+\fBTIP:\fP
+.INDENT 0.0
+.INDENT 3.5
+A DNSSEC\-validating resolver can be set as a parent.
+.UNINDENT
+.UNINDENT
 .SS check\-interval
 .sp
 Interval for periodic checks of DS presence on parent\(aqs DNS servers, in the

doc/reference.rst

@@ -603,6 +603,9 @@ rollover must be pushed forward manually.
 
 *Default:* not set
 
+.. TIP::
+   A DNSSEC-validating resolver can be set as a parent.
+
 .. _submission_check-interval:
 
 check-interval