open_memstream: update docs wrt posix issue 8

[DJ Delorie]

Collin Funk <collin.funk1@gmail.com> writes:
> But I mentioned the typos because they were in the commit message. Of
> course typos are no big deal, but we can always fix them before pushing
> to make the git log easier to read.

I'm not explaining then.  The commit message is:

    manual: Explain our implementation-defined memstream semantics
    
    Posix Issue 8 adds an implementation-defined item we don't already
    cover, about seeking backwards.  This defines our implentation.
    
    https://issues.redhat.com/browse/RHEL-3008
    https://pubs.opengroup.org/onlinepubs/9799919799/functions/open_memstream.html