gb18030(5) manual page for review

[Kang Liu]

Hi, Tim Robbins
	Thanks for writing manual page for gb18030 :)
	I think there are 2 points need to be improved. 
1.why not use  "GB2312-1980" instead of  "GB2312-80"  (more official and Y2K
compatible? ^_^); 
2."GB18030 is a superset of the older GB2312-80 and GBK encodings" that is
right, but for more exactly GB18030 is not only include GB2312 and GBK but
also contains all CJK Extension(Chinese, Japanese,Korean unification
character set).

Regards,

Kang Liu