Update documentation

docbook/Admin_Guide/en-US/config/duedate.xml

@@ -13,13 +13,15 @@
 				</para>
 			</listitem>
 		</varlistentry>
+
 		<varlistentry>
 			<term>$g_due_date_view_threshold</term>
 			<listitem>
 				<para>Threshold to see due date. Default is NOBODY.
 				</para>
 			</listitem>
 		</varlistentry>
+
 		<varlistentry>
 			<term>$g_due_date_default</term>
 			<listitem>
@@ -30,5 +32,37 @@
 				</para>
 			</listitem>
 		</varlistentry>
+
+		<varlistentry>
+			<term>$g_due_date_warning_levels</term>
+			<listitem>
+				<para>Due date warning levels. A variable number of Levels
+					(defined as a number of seconds going backwards
+					from the current timestamp, compared to an issue's due date) can be defined.
+					Levels must be defined in ascending order.
+					<itemizedlist>
+						<listitem>
+							<para>The first entry (array key 0) defines <emphasis>Overdue</emphasis>.
+								Normally and by default, its value is <literal>0</literal>, meaning
+								that issues will be marked overdue as soon as
+								their due date has passed. However, it is also possible to set it to a
+								higher value to flag overdue issues earlier, or even use a negative value
+								to allow a "grace period" after due date.
+							</para>
+						</listitem>
+						<listitem>
+							<para>Array keys 1 and 2 offer two levels of <emphasis>Due soon</emphasis>:
+								orange and green.
+								By default, only the first one is set, to 7 days.
+							</para>
+						</listitem>
+					</itemizedlist>
+					Out of the box, MantisBT allows for 3 warning levels. Additional ones may
+					be defined, but in that case new <literal>due-N</literal> CSS rules
+					(where N is the array's index) must be created otherwise the extra levels
+					will not be highlighted in the UI.
+				</para>
+			</listitem>
+		</varlistentry>
 	</variablelist>
 </section>