[5381] Explain why use explciit subnet identifiers with host reservations.

doc/guide/dhcp4-srv.xml

@@ -3829,7 +3829,8 @@ subnets belonging to the same shared network.</para>
 <para>While not strictly mandatory, it is strongly recommended to use explicit
 "id" values for subnets if you plan to use database storage for host
 reservations. If ID is not specified, the values for it be autogenerated,
-i.e. it will assign increasing integer values starting from 1.</para>
+i.e. it will assign increasing integer values starting from 1. Thus, the
+autogenerated IDs are not stable across configuration changes.</para>
     </section>
 
   </section>

doc/guide/dhcp6-srv.xml

@@ -3398,7 +3398,9 @@ subnets belonging to the same shared network.</para>
 <para>While not strictly mandatory, it is strongly recommended to use explicit
 "id" values for subnets if you plan to use database storage for host
 reservations. If ID is not specified, the values for it be autogenerated,
-i.e. it will assign increasing integer values starting from 1.</para>
+i.e. it will assign increasing integer values starting from 1. Thus, the
+autogenerated IDs are not stable across configuration changes.
+</para>
     </section>
 
   </section>