[PATCH] expand and clarify help for update

[Paul R]

Adrian> Ok. I'll drop working on this. Please ignore that patch.

I consider your proposed version to be overall better than the
previous one, it reads better and contain more information (inluding
the very important reference to 'hg update null'). I personnaly like
to have rich documentation in my terminal, and don't mind to pipe very
long things in a pager. But some other people like things to be short,
so we must find an agreement on that. As a naive solution, I would
invite them to propose some replacement for paragraphs they find too
long. The only constraint is to keep the same amount of useful
information. What do others think ?

-- 
  Paul