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

[Collin Funk]

Florian Weimer <fweimer@redhat.com> writes:

> This looks really good to me, thanks!
>
> Reviewed-by: Florian Weimer <fweimer@redhat.com>
>
> Please give Collin a chance to re-confirm the previous Reviewed-by
> before pushing.

Tested the HTML and PDF.

Reviewed-by Collin Funk <collin.funk1@gmail.com>

Thanks for the suggestion Florian and v2 patch DJ.

Collin