docs: another parallel make issue

docs: another parallel make issue

* doc/autoconf.texi (Parallel Make): Document that some make
implementations, when run in parallel mode, connect stdout and
stderr of child processes to pipes or temporary files, and might
re-route stderr of spawned processes to stout.  Also document
that FreeBSD make in parallel mode reuses the same shell for
multiple commands within one recipe (like NetBSD make does).