[ADD] Documentation update in base_trainer.py

[theodorju]

Types of changes

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

Description

I have updated the documentation of base_trainer for #406 in places where it seemed reasonable to add documentation.

Motivation and Context

Changes fix #406. This pull request includes the changes after the comments on PR 447 which reported additional changes due to rebase.

How has this been tested?

Since the changes are only in the documentation, I have not executed any tests for this issue.

[ravinkohli]

Thanks for this PR. I think it can be merged.