Mention deprecation of implicit nullable types in function docs (#4008)

Author: Girgias

language/functions.xml

@@ -454,8 +454,9 @@ Making a bowl of raspberry natural yogurt.
      dropping the default value, since it will never be used.
      One exception to this rule are arguments of the form
      <code>Type $param = null</code>, where the &null; default makes the type implicitly
-     nullable. This usage remains allowed, though it is recommended to use an
-     explicit <link linkend="language.types.declarations.nullable">nullable type</link> instead.
+     nullable. This usage is deprecated as of PHP 8.4.0, and an explicit
+     <link linkend="language.types.declarations.nullable">nullable type</link>
+     should be used instead.
      <example>
       <title>Declaring optional arguments after mandatory arguments</title>
       <programlisting role="php">
@@ -464,7 +465,7 @@ Making a bowl of raspberry natural yogurt.
  function foo($a = [], $b) {} // Default not used; deprecated as of PHP 8.0.0
  function foo($a, $b) {}      // Functionally equivalent, no deprecation notice
 
- function bar(A $a = null, $b) {} // Still allowed; $a is required but nullable
+ function bar(A $a = null, $b) {} // Deprecated as of PHP 8.4.0; $a is required but nullable
  function bar(?A $a, $b) {}       // Recommended
  ?>
  ]]>