bug#64960: Documentation for copy-sequence

[Drew Adams <drew.adams@oracle.com>]

> > > > When I modify the copy, the original remains intact.
> > >
> > > No, if you modify an element of the copy, the corresponding element of
> > > the original is also changed. That's what "shared" means.
> >
> > Except for strings
> 
> It is irrelevant what happens when you modify elements of a copied
> string because it is impossible.  The statement is simply not applicable
> to strings.  That's why the docstring only speaks of "list, vector or
> record".
> 
> Note that substituting a character in a string is not the same as
> modifying the character.  You are substituting a sequence element with
> another one and not modifying an element in that case.
> 
> The same is true for the list (1 2 3) for example.  Exactly the same
> situation as for a string: You can replace the number 2 in a copy with
> another one but you can't modify the number 2 in the list.  Numbers and
> characters are not mutable.

Well put.