Fix clock instructions updated

[Daniel Drake]

Having seen a handful of RTC battery problems in Nicaragua I found
myself revisiting the process, and struggling to explain the wiki page
to the team.

So I restructured it a bit, mainly to take the focus away from the
problems caused by ancient firmware versions and badly shaped holders,
and instead to focus on the problems caused by battery leakage and
defunct batteries (the common cases nowadays).

I also added some information about the XO-1.5 with help from Paul

http://wiki.laptop.org/go/Fix_Clock

Review appreciated..and one detail we're not 100% sure on: does the
XO-1.5 continue to use exactly the same RTC battery as the XO-1?

Thanks,
Daniel