Documentation of collection.py can be improved

[jaw111]

Referring to: https://rdflib.readthedocs.io/en/stable/apidocs/rdflib.html#rdflib.collection.Collection

Suggestions to improve:

• Show how a Collection can be constructed by passing a list as the third argument when instantiating the Collection class, instead of using g.add to iteratively add statements
• The example for append method does not show how to append an item to a collection
• The example for __dict__ is totally messed up