Add documentation about which PHP versions are documented (#3700)

Author: jimwins

preface.xml

@@ -24,6 +24,16 @@
   and other <link linkend="appendices">supplemental</link> 
   information.
  </para>
+ <simpara>
+  This manual includes information covering the two most recent major
+  releases of PHP (versions 7 and 8). Information about extensions
+  that are no longer included with those versions of PHP and changes to
+  functions in earlier versions is not included. Archives of older
+  versions of the manual that document previous releases of PHP are
+  available from the "More documentation" section on the
+  <link xlink:href="&url.php.docs;">"Documentation" page of the PHP
+  website</link>.
+ </simpara>
  <para>
   You can download this manual in several formats at <link
   xlink:href="&url.php.doc.downloads;">&url.php.doc.downloads;</link>.