Add EXAMPLES-section entry for new key+=append syntax of sysrc(8).

MFC after:	3 days
X-MFC-to:	stable/10 stable/9
X-MFC-with:	274068
Thanks to:	@SeanChittenden


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274119 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f