man/emerge.1: --quiet-build=n overridden by --jobs (505116)

Users may be confused that --quiet-build=n has no effect when used
together with --jobs, so it deserves to be mentioned in the man page.

X-Gentoo-Bug: 505116
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=505116
Acked-by: Alexander Berntsen <bernalex@gentoo.org>