ext/pcre: document variable-length lookbehind and longer subpattern names (PHP 8.4)

reference/pcre/pattern.syntax.xml

@@ -1510,6 +1510,10 @@
    also by name. There are two alternative syntaxes
    <literal>(?&lt;name&gt;pattern)</literal> and <literal>(?'name'pattern)</literal>.
   </para>
+  <simpara>
+   As of PHP 8.4.0, which bundles PCRE2 10.44, the maximum length of a
+   subpattern name is 128 characters; prior to PHP 8.4.0 it was 32 characters.
+  </simpara>
 
   <para>
    Sometimes it is necessary to have multiple matching, but alternating
@@ -1915,6 +1919,14 @@
    at the ends of strings; an example is given at the end
    of the section on once-only subpatterns.
   </para>
+  <simpara>
+   The fixed-length restriction described above applies prior to PHP 8.4.0.
+   As of PHP 8.4.0, which bundles PCRE2 10.44, a lookbehind assertion may match
+   strings of variable length: alternatives of differing lengths (such as
+   <literal>(?&lt;!dogs?|cats?)</literal>) and top-level branches that can match
+   more than one length (such as <literal>(?&lt;=ab(c|de))</literal>) are now
+   permitted, up to an implementation-defined maximum length.
+  </simpara>
   <para>
    Several assertions (of any sort) may occur in succession.
    For example,