Description:
------------
The description of the "output_callback" parameter in the doc for ob_start() states that:
  "The function will be called when ob_end_flush() is called, or when the output buffer is flushed to the browser at the end of the request."
  
The testcase below shows 3 undocumented cases where the callback is invoked:
- when the output buffer is flushed with ob_flush()
- when the output buffer is cleaned with ob_clean() 
- when the output buffer is cleaned with ob_end_clean().

This could be indicated by adapting the description as follows:
"The callback function will be invoked when the output buffer is flushed explicitly (using ob_flush() or ob_end_flush()), when it is flushed implicitly at the end of the request, or when it is cleaned (using ob_clean() or ob_end_clean()). In the case where the buffer is cleaned, the callback's return value is ignored."
 

Reproduce code:
---------------
<?php
$a = '';
function output_handler($buffer, $mode) {
	global $a;
	static $i=0;
	$i++;
	$a .= "\n-->$i: In output_handler($buffer, $mode)";
	return "Hello";
}

echo "About to start buffer.\n";
ob_start('output_handler');
echo "About to call 'ob_flush()'";
ob_flush();
echo "About to call 'ob_clean()'";
ob_clean();
echo "About to call 'ob_end_clean()'";
ob_end_clean();
echo "\nEnded buffer.\n";

echo "\n\nShow when the callback was really invoked:";
echo $a;
?>

Expected result:
----------------
N/A

Actual result:
--------------
About to start buffer.
Hello
Ended buffer.


Show when the callback was really invoked:
-->1: In output_handler(About to call 'ob_flush()', 3)
-->2: In output_handler(About to call 'ob_clean()', 2)
-->3: In output_handler(About to call 'ob_end_clean()', 4)