clarify C++ copy constructor vs assignment operator

[chmullig]

Clarify what happens in each of the situations below.

MyString s1("hello");
MyString s2(s1);
MyString s3 = s1;
MyString s4;
s4 = s1;

[kevin1]

Additionally, the example of returning a string by value should be removed because it's no longer accurate in C++11. The code would invoke std::basic_string's move constructor instead.

string call_copy(string myString) //copy constructor is called to create temporary copy of myString local to call_copy
{
    return myString; //copy constructor is called to return myString by value
}

We might be ok to remove the bad example after adding Chris's suggestion above.

[hmontero1205]

WIP: #154