docs/101271: serial console documentation implies kernel rebuild required -- doesn't mention device hints

[dru at FreeBSD.org]

Synopsis: serial console documentation implies kernel rebuild required -- doesn't mention device hints

Responsible-Changed-From-To: freebsd-doc->dru
Responsible-Changed-By: dru
Responsible-Changed-When: Thu May 8 16:09:59 UTC 2014
Responsible-Changed-Why: 
I'll take

http://www.freebsd.org/cgi/query-pr.cgi?pr=101271