4081: Bug 1572796 - Improve service-experiments public API docstrings r=travis79 a=travis79
As part of Doc Days of Summer 2019 2.0, improves the docstrings of the public API classes and especially the exposed methods and properties.
### Pull Request checklist
<!-- Before submitting the PR, please address each item -->
- [ ] **Quality**: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
- [ ] **Tests**: This PR includes thorough tests or an explanation of why it does not
- [ ] **Changelog**: This PR includes [a changelog entry](https://github.com/mozilla-mobile/android-components/blob/master/docs/changelog.md) or does not need one
- [ ] **Accessibility**: The code in this PR follows [accessibility best practices](https://github.com/mozilla-mobile/shared-docs/blob/master/android/accessibility_guide.md) or does not include any user facing features
### After merge
- [ ] **Milestone**: Make sure issues closed by this pull request are added to the [milestone](https://github.com/mozilla-mobile/android-components/milestones) of the version currently in development.
- [ ] **Breaking Changes**: If this is a breaking change, please push a draft PR on [Reference Browser](https://github.com/mozilla-mobile/reference-browser) to address the breaking issues.
Co-authored-by: Travis Long <[email protected]>
