Add example showing explicit unicode for text values in entities. (#5263)

Closes #5262.

Author: tseaver

packages/google-cloud-datastore/google/cloud/datastore/__init__.py

@@ -25,6 +25,7 @@
    >>> key
    <Key('EntityKind', 1234), project=...>
    >>> entity = datastore.Entity(key)
+   >>> entity['question'] = u'Life, universe?'  # Explicit unicode for text
    >>> entity['answer'] = 42
    >>> entity
    <Entity('EntityKind', 1234) {'answer': 42}>