Fix GH-2172 (Remove opcache.fast_shutdown from OPcache installation page)

We do not remove it yet, but rather add a respective comment.

Author: cmb69

reference/opcache/configure.xml

@@ -43,7 +43,7 @@ opcache.memory_consumption=128
 opcache.interned_strings_buffer=8
 opcache.max_accelerated_files=4000
 opcache.revalidate_freq=60
-opcache.fast_shutdown=1
+opcache.fast_shutdown=1 ; prior to PHP 7.2.0
 opcache.enable_cli=1
 ]]>
    </programlisting>