thoroughly iterate all scenarios where preloading is invalidated

[cianbuckley]

What are you trying to accomplish?

Follow up from this PR

Goal is to thoroughly document when:

• the SDK invalidates preloading
• the merchant developer is required to invalidate
• How it works preloading works internally

Before you merge

Testing

• [ ] I've added tests to support my implementation

Contribution guidelines

• [x] I have read and agree with the Contribution Guidelines.
• [x] I have read and agree with the Code of Conduct.

Documentation

• [x] I've updated any documentation related to these changes.
• [x] I've updated the README.

Versioning

• [ ] I have bumped the version number in the .podspec file.
• [ ] I have bumped the version number in the README.
• [ ] I have added a Changelog entry.

After merging

If your changes required a version bump, please add an entry under the Releases page - doing so will trigger a CI workflow to publish the latest Cocoapod.