Fix #77477: Document hard_timeout

I've encountered issues relating to the hard timeout, so having
this explained for myself in a few months time and others will be
handy.

ML post where hard_timeout was established (not linked in docs, but
may be useful):

http://php-news.ctrl-f5.net/message/php.internals/92559

Closes GH-491.

appendices/ini.core.xml

@@ -262,6 +262,19 @@
       </term>
       <listitem>
        <para>
+        When the timeout set in <link linkend="ini.max-execution-time">max_execution_time</link>
+        has been hit, the PHP runtime will tear down resources gracefully. If
+        something gets stuck while this happens, the hard timeout will tick
+        for the set amount of seconds. When the hard timeout is hit, PHP will
+        exit ungracefully. When set to 0, the hard timeout will never activate.
+       </para>
+       <para>
+        When PHP stops from a hard timeout, it will look something like this:
+        <screen>
+<![CDATA[
+Fatal error: Maximum execution time of 30+2 seconds exceeded (terminated) in Unknown on line 0
+]]>
+        </screen>
        </para>
       </listitem>
      </varlistentry>