Weak*Dictionary: Added docstrings to the classes.

Weak*Dictionary:  Added docstrings to the classes.

Weak*Dictionary.update():  No longer create a temporary list to hold the
    things that will be stuffed into the underlying dictionary.  This had
    been done so that if any of the objects used as the weakly-held value
    was not weakly-referencable, no updates would take place (TypeError
    would be raised).  With this change, TypeError will still be raised
    but a partial update could occur.  This is more like other .update()
    implementations.

Thoughout, use of the name "ref" as a local variable has been removed.  The
original use of the name occurred when the function to create a weak
reference was called "new"; the overloaded use of the name could be
confusing for someone reading the code.  "ref" used as a variable name
has been replaced with "wr" (for 'weak reference').