document the *MARK verb in PCRE

Author: chx

reference/pcre/pattern.syntax.xml

@@ -1311,6 +1311,17 @@
    rest of the main pattern as well as the alternative in the
    subpattern.
   </para>
+  <para>
+   It is possible to record which alternative was matched by using 
+   <literal>(*MARK:NAME)</literal> or <literal>(*:NAME)</literal>.
+   Any number of such <literal>(*MARK)</literal> verbs may appear and their
+   names do not have to be unique. When a match succeeds, the name of the
+   last-encountered <literal>(*MARK:NAME)</literal> will be placed among
+   matches as if it were a capture group called <literal>MARK</literal> so it
+   can be read from the <parameter>matches</parameter> of
+   <function>preg_match</function> and will be passed to the
+   <parameter>callback</parameter> of <function>preg_match_all</function> etc.
+  </para>
  </section>
 
  <section xml:id="regexp.reference.internal-options">