Update docstring for 'Connection.save_entity'.

Indicate that upsert clears existing properties before applying those
passed to the API.

Fixes #167.
Update doctstrings for 'Entity.save' and 'Connection.save_entity'.

Reinforce that previously-stored properties which don't have values set
in the current entity instance will be removed.
Merge pull request #258 from tseaver/167-connection_save_entity

Fix #167:  Update docstring for 'Connection.save_entity'.