[PATCH] expand and clarify help for update

[Greg Ward]

On 30 October 2008, Andrei Vermel said:
> I think the current help text for 'update'  is too many words.
> It requires reading into it, it's hard to make sense at a glance.

Maybe that's just a symptom that 'update' is a complex, powerful, and
rather tricky command.  When the documentation is too complex to follow,
it might mean that the thing being described is just too complex.

Maaaaaybe 'update' is trying to do too much with one command.  It's not
quite as bonkers as "git checkout", but so far in my experience with
Mercurial, it seems to be the most complex and subtle command ... or at
least, the most complex and subtle command that is used regularly.

        Greg