rseq notes from Cauldron

[Florian Weimer]

* Mathieu Desnoyers:

>>> We need a little bit of documentation for the manual, mentioning the
>>> new symbol and the public header.  Likewise, this should be part of
>>> the patch.
>
> I suspect I need to add something under glibc manual/*.texi . Is there
> a preferred existing manual file for this, or should I add a new one ?

At the end of manual/threads.texi seems fine to me.