doc: add warning about not synchronized zone file

doc/man/knot.conf.5in

@@ -878,6 +878,15 @@ the immediate sync with a zone file is not desirable, increase the value.
 .UNINDENT
 .UNINDENT
 .sp
+\fBWARNING:\fP
+.INDENT 0.0
+.INDENT 3.5
+If the zone file is not up\-to\-date, the zone should be flushed before
+editation or the SOA record must be untouched after editation. Otherwise
+the journal can\(aqt be applied.
+.UNINDENT
+.UNINDENT
+.sp
 \fIDefault:\fP 0 (immediate)
 .SS ixfr\-from\-differences
 .sp

doc/reference.rst

@@ -1024,6 +1024,11 @@ can be used (manual zone flush is still possible).
    If you are serving large zones with frequent updates where
    the immediate sync with a zone file is not desirable, increase the value.
 
+.. WARNING::
+   If the zone file is not up-to-date, the zone should be flushed before its
+   zone file editation or the SOA record must be untouched after editation.
+   Otherwise the journal can't be applied.
+
 *Default:* 0 (immediate)
 
 .. _zone_ixfr-from-differences: