Documentation/unicode.txt: convert it to ReST markup

Probably, unicode is something that we might remove from the
docs, as all modern systems support it. Yet, this chapter
is fun, as it mentions support for the Klington fictional
charset ;)

On the other hand, I bet all other OS user manuals
explicit mention unicode support.

So, convert it to ReST and include it at the user's book.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>