tensorflow_probability docstring_util location

The location of the docstring_util in tensorflow_probability 0.8.0 has changed, resulting in edward2 errors such as:
```
File "/home/wibble/.conda/envs/gem/lib/python3.7/site-packages/edward2/generated_random_variables.py", line 26, in
from tensorflow_probability.python.util import docstring as docstring_util
ImportError: cannot import name 'docstring' from 'tensorflow_probability.python.util'
```

Start by inspecting generated_random_variables.py and the failing import from tensorflow_probability.python.util shown in the traceback. Reproduce the ImportError with TensorFlow Probability 0.8.0 and trace the current docstring_util location. Done means edward2 imports successfully without the reported error.

Written by the indexing model from the issue text.