chore: add docstrings on datastore type utils

[svidgen]

Description of changes

Add docstrings on the exposed type util functions so that they're represented and/or hidden in customer-facing docs correctly.

For most of these utils, that means explicitly marking them as @private to hide them from docs generation. It seemed easy enough to add a little context on the meaning of the return types for most of these as well, so that's included, but should be hidden from customer docs -- internal context only.

syncExpression is intended for customer consumption, so a description intended for customer consumption with sample code (borrowed from docs site) has been added.

Issue #, if available

Description of how you validated changes

Docstring changes only.

Checklist

• PR description included
• yarn test passes
• Unit Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• New source file paths included in this PR have been added to CODEOWNERS, if appropriate

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.