charset documentation patches

[Markus Kuhn]

I just had a quick look at charset.texi and noticed quite a number of
minor mistakes that I fixed. Mostly UCS4 -> UCS-4 (that how it is
written in all the standards), second amendment -> Amendment 1 (there
was no second amendment to ISO C90), etc. I also slightly modernized the
description of the relationship between Unicode and ISO 10646.

Patch attached.

Markus

-- 
Markus G. Kuhn, Computer Laboratory, University of Cambridge, UK
Email: mkuhn at acm.org,  WWW: < http://www.cl.cam.ac.uk/~mgk25/ >