[Train] Clarify Session.get_next docstring

[bveeramani]

Why are these changes needed?

This PR addresses two issues an issue with the Session.get_next docstring:

• The docstring summary is in the wrong tense. It's unclear whether the tense should be imperative or non-imperative. The Ray documentation states that we use Google style (which is non-imperative), but we are formatting using PEP8 (which is imperative). Moreover, we use both imperative and non-imperative summaries across the Ray Train code. I've stuck with a non-imperative summary for consistency with the rest of the Session class.
• The docstring doesn't describe the conditions under which the function returns None.

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[bveeramani]

Wasn't sure whether to put this information under Returns: or under the docstring summary. I don't think it really matters, though; I may just be bike-shedding.