[issue9021] no copy.copy problem description

[Senthil Kumaran]

I am +0 on this change. The existing introduction is fine and the
discussion where shallow and deep copy is discussed is more important.

It is okay to assume that the reader is aware of the assignment
operation nature, i.e. it does not create a copy and you need this
module.  

The doc change suggested by Terry could be helpful in the tutorial
where a copy module is mentioned.