Misleading `window-system' docstring (was Re: Dashes in mode-line)

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Eli Zaretskii <address@hidden> writes:

>> From: Julien Danjou <address@hidden>
>> Cc: address@hidden
>> Date: Tue, 19 Oct 2010 12:32:33 +0200
>> 
>> +         `(:eval (unless window-system
>> +                   ,(propertize "-%-" 'help-echo help-echo)))))
>
> Using window-system is not right here, because the DOS port defines a
> non-nil value, but uses a text terminal.

Could you please fix the docstring of `window-system' to reflect that?
It seems to be widely used that (incorrect, as you point out) way, and
its docstring is pretty clear (and thus incorrect) on the matter:

  The value is nil if the selected frame is on a text-only-terminal.

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