Updated docs according to the changes from the previous commit

docs/en/language-snippets.ent

@@ -12,6 +12,17 @@
  </para>
 </note>'>
 
+<!ENTITY note.parameter.function '<note>
+ <title>Technical Note</title>
+ <para>
+  If the chosen <parameter>function</parameter> callback is of the form 
+  <literal>array(&amp;$object, $method)</literal>, only one instance of the
+  same class and with the same <literal>$method</literal> can be registered. The
+  latest registered <parameter>function</parameter> callback will be used in 
+  such a scenario.
+ </para>
+</note>'>
+
 <!ENTITY parameter.compileid '<para>
  As an optional third parameter, you can pass a
  <parameter>$compile_id</parameter>.

docs/en/programmers/api-functions/api-register-outputfilter.xml

@@ -21,7 +21,7 @@
    for more information on how to set up an output filter function.
   </para>
    &api.register.snippet;
-
+   &note.parameter.function;
 <para>
 See also
 <link linkend="api.unregister.outputfilter"><varname>unregister_outputfilter()</varname></link>,

docs/en/programmers/api-functions/api-register-postfilter.xml

@@ -19,7 +19,7 @@
    more information on how to setup a postfiltering function.
   </para>
    &api.register.snippet;
-
+   &note.parameter.function;
   <para>
   See also
   <link linkend="api.unregister.postfilter">

docs/en/programmers/api-functions/api-register-prefilter.xml

@@ -19,6 +19,7 @@
    more information on how to setup a prefiltering function.
   </para>
  &api.register.snippet;
+ &note.parameter.function;
 
 
     <para>