[v2] manual: Explain our implementation-defined memstream semantics

[DJ Delorie]

Collin Funk <collin.funk1@gmail.com> writes:
> Florian Weimer <fweimer@redhat.com> writes:
>> Reviewed-by: Florian Weimer <fweimer@redhat.com>
> Reviewed-by Collin Funk <collin.funk1@gmail.com>

Thanks!
Pushed.