isalpha man page - return value in case of argument equal to EOF

[Florian Weimer]

* Andreas Schwab:

> On Jun 12 2023, Florian Weimer wrote:
>
>> * Andreas Schwab via Libc-alpha:
>>
>>> On Jun 12 2023, Luigi Pellecchia via Libc-alpha wrote:
>>>
>>>> Even if there may be several reasons to accept EOF, it is not clear what
>>>> the expected value should be in that case.
>>>
>>> Since EOF is not a character, it cannot be part of any of the character
>>> classes.
>>
>> EOF can be equal to 'ÿ' (as in, have the same value and type).
>
> No.  EOF is -1, (unsigned char) 'ÿ' is 255.

We currently do not document the (unsigned char) part in the manual.

>> It's unclear to me what we actually want to document in terms of C
>> extensions.  I thought that is* was unspecified for EOF,
>
> The classification functions are required to accept EOF.

True, but I don't think the standard specifies the result.  But that
doesn't really matter.

Should we document what we actually implement in the manual?

Thanks,
Florian