[patch v4] manual: Document more sigaction flags

[Florian Weimer]

* DJ Delorie:

> v4: Correct SA_RESETHAND default.
>
> -- >8 --
>
> Adds documentation for three-argument handler
>
> Adds remainder of the SA_* flags

This version looks okay to me.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

Thanks,
Florian