apt_preferences(5): Mention overlapping of pin matches

doc/apt_preferences.5.xml

@@ -123,6 +123,9 @@ from archives which in their <filename>Release</filename> files are marked as "N
 <listitem><simpara>to the versions that belong to the target release.</simpara></listitem>
 </varlistentry>
 </variablelist>
+
+The highest of those priorities whose description matches the version is assigned to the
+version.
 </para>
 
 <para>If the target release has not been specified then APT simply assigns