It is also undocumented for expr3.

Note that the use of multiple expressions in expr3 was already mentionned in example4.

Anyway, here is a suggestion of a possible improvement:

http://patches.colder.ch/control-structures-for.patch

Thanks in advance.

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.

thanks for the patch

I thank you sincerely for the answer.

You can use a comma in expr2 too, it's equivalent to || but much lower in precedence.

Thanks for the note arpad, 

here is the updated patch:

http://patches.colder.ch/control-structures-for-2.patch

IMHO
(1) should only be documented what is intended. ZEND should say about comma in expr2 before it makes it into the manual. Such deviation from C should be intentional if at all.
(2) in C++, by 5.2 of its definition, only assigments can be comma-sequenced. Simple boolean expressions are not yet assignments. The value of a sequenced expression is the value of the last assigment, not an OR of them. 
(3) it would be more logical to give comma in expr-2 the meaning of AND: continue while (data available AND buffer available). But better nothing.

It is however true that it executes as arpad described.

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.

fixed for months :]