Properly format manual sample code

[bjwatson]

For example, see http://pythonhosted.org/gax-google-pubsub-v1/google.protobuf.timestamp_pb2.html#module-google.protobuf.timestamp_pb2. This manually written sample code is poorly formatted in the Python doc.

We will probably need some kind of hint in the proto file that it's sample code, such as the markdown conventional ```.

[bjwatson]

I fixed the public link above to render properly, by manually adding '::' to the line before each code sample. This issue is for eventually automating this, instead of manually tweaking the auto-gen results.

[bjwatson]

The proto comments is markdown, so four leading spaces is our hint that it's code.

[bjwatson]

Parent issue is googleapis/gapic-generator#331

[software-dov]

Out of scope for microgenerator.