Improve the description of big5(5)

• Fix the statement that big5 is a de facto standard of Traditional Chinese text [1]
• Add a BUGS section describes the problem of big5 and suggests use utf8

PR: 189095
Submitted by: Brennan Vincent <brennan@umanwizard.com> [1]
Reviewed by: Ting-Wei Lan <lantw44@gmail.com>
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21622