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

Indicate that upsert clears existing properties before applying those passed to the API.

Fixes #167.
This is a little misleading since the properties aren't actually being cleared. Suggest:

> This will overwrite an existing entity stored with the same key. An entity
> will be completely overwritten. For example if
>    <Entity[{'kind': 'some-kind'}] {'a': 1, 'b': 2}>
> was stored and you overwrite with the single property `b = 3`, the newly
> stored entity will be
>    <Entity[{'kind': 'some-kind'}] {'b': 3}>
> because the full entity was overwritten (i.e. no untouched properties
> stick around.)