[master] Merge trac4101 - documentation for classification

Add the classify.xml file to document the client classification
system

Update the dhcp4-srv.xml and dhcp6-srv.xml files to provide a brief
description of the classification system and pointers to the
text in classify.xml

diff --cc ChangeLog
index 508d48ef85d37ede07c7c24235e5ffd1a45a5406,26e89f309444bc7327e41f39112a68c8113d7088..034ce6a980d4ceec619682960f340b70e4a7ff50
--- 1/ChangeLog
--- 2/ChangeLog
+++ b/ChangeLog
@@@ -1,28 -1,8 +1,33 @@@
 -1XXX. [doc]           [sar]
++1056. [doc]           [sar]
+       Added description of the expression based classification
+       system to the Kea Admin Guide.
+       (Trac #4101     git <tbd>)
+ 
 +1055. [func]          [fdupont]
 +      Classify match expressions are evaluated on incoming packets and
 +      requested options are appended when configured by the subnet, a
 +      class or globally.
 +      (git #409[78], git xxx)
 +
 +1054. [func]          [tmark]
 +      Replaced underscores, "_", with hyphens "-", in the parameter
 +      names used in the kea-ddns server's configuration as well as
 +      those in the DDNS messages (NCRs) sent to the server. For
 +      example, "ip_address" is now "ip-address", "change_type" is
 +      now "change-type".  This makes JSON element naming consistent
 +      throughout Kea.
 +      (Trac #4202, git 91bf527662060d4b1e294cd53e79b431edf0e910)
 +
 +1053. [doc]           tomek
 +      Support for DHCPDECLINE (v4) and DECLINE (v6) messages is
 +      now described in the Kea User's Guide.
 +      (Trac #3990, git 6b0db91610420f3cce55d9f887d7c73660ece209)
 +
 +1052. [func]          marcin
 +      libeval: expressions involving options can now use textual or
 +      hexadecimal format of the options.
 +      (Trac #4093, git 4cdf0fff1067b3dde6570dc6831e8b1343bc50fe)
 +
  1051. [func]          [tmark]
        kea-dhcp4 and kea-dhcp6 configuration parsing now supports
        the "client-classes" element for defining client classes.

diff --cc doc/guide/dhcp4-srv.xml
Simple merge

diff --cc doc/guide/dhcp6-srv.xml
Simple merge