doc: add note about RRSIG inception setting

doc/man/knot.conf.5in

@@ -822,6 +822,14 @@ Has infuence over ZSK key lifetime.
 .sp
 A validity period of newly issued signatures.
 .sp
+\fBNOTE:\fP
+.INDENT 0.0
+.INDENT 3.5
+The RRSIG\(aqs signature inception time is set to 90 minutes in the past. This
+time period is not counted to the signature lifetime.
+.UNINDENT
+.UNINDENT
+.sp
 \fIDefault:\fP 14 days
 .SS rrsig\-refresh
 .sp

doc/reference.rst

@@ -914,6 +914,10 @@ rrsig-lifetime
 
 A validity period of newly issued signatures.
 
+.. NOTE::
+   The RRSIG's signature inception time is set to 90 minutes in the past. This
+   time period is not counted to the signature lifetime.
+
 *Default:* 14 days
 
 .. _policy_rrsig-refresh: