Spanner: Fix run_in_transaction return value docs

[wchargin]

Summary:
These docs used to be correct, but Session.run_in_transaction was
changed from returning the commit timestamp to returning the function
value in #3753, and the upstream docs were not updated.

The docs for run_in_transaction are now identical across Database
and Session.

The tests for run_in_transaction (in test_database.py) are wrong, as
they mock out the session object instead of using a real session, and
they also were not updated in #3753. This change does not fix them.

Test Plan:

$ virtualenv -q -p python3.6 ./ve
$ . ./ve/bin/activate
(ve) $ pip install -q google-cloud-spanner==1.10.0
(ve) $ pip freeze | grep google-cloud-spanner
google-cloud-spanner==1.10.0
(ve) $ cat test.py; echo
from google.cloud import spanner_v1

client = spanner_v1.Client()
instance = client.instance("my-instance-name")
database = instance.database("my-database-name")
result = database.run_in_transaction(lambda txn: "ahoy")
print(result)

(ve) $ python test.py
ahoy

wchargin-branch: run-in-transaction-rval

[tseaver]

@wchargin Thanks for the patch!