bug#24287: Process filters received stderr output, not documented

[Eli Zaretskii <eliz@gnu.org>]

> Cc: Eli Zaretskii <eliz@gnu.org>
> From: Clément Pit--Claudel <clement.pit@gmail.com>
> Date: Mon, 22 Aug 2016 14:38:01 -0400
> 
>        The output that a subprocess writes to its standard output stream is
>                                                   ^^^^^^^^^^^^^^^
>     passed to a function called the “filter function”.  The default filter
>     function simply inserts the output into a buffer, which is called the
>     associated buffer of the process (*note Process Buffers::).  If the process
>     has no buffer then the default filter discards the output.
> 
> The highlighted section is not true, is it?

Of course, it's true: the standard output _is_ passed to a filter.