Document the general effect of the comma operator

Closes: #574939

doc/apt_preferences.5.xml

@@ -254,6 +254,11 @@ Pin-Priority: 500
 </programlisting>
 </listitem>
 </itemizedlist>
+
+The effect of the comma operator is similar to an "and" in logic: All
+conditions must be satisfied for the pin to match. There is one exception:
+For any type of condition (such as two "a" conditions), only the last such
+condition is checked.
 </para>
 
 </refsect2>