[Hydrogen docs]: Clarify behavior of initialVariantId prop

[mcvinci]

This PR:

• Augments the description of the initialVariantId prop to clarify behavior
• Relates to Adopt Remix v2_errorBoundary future flag hydrogen#729

Tophatted

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset.
If the changes are user-facing and should cause a version bump, run "yarn changeset add" to track your changes and include them in the next release CHANGELOG.
If you are making simple updates to examples or documentation, you do not need to add a changeset.

[mcvinci]

@frehner I've made an attempt to capture the logic you referenced in this issue. Would you mind taking a look and confirming that what we're saying here is accurate? 🙏

Also, do we have any guidance that we can share at this point in terms of how to unset initialVariantId? I'd like to capture this in the reference topic as well!

[frehner]

Would you mind taking a look and confirming that what we're saying here is accurate? 🙏

That was the intention, yes, so I think you're good. However, @cartogram was the one that actually did the work on this, so he gets all the credit.

Also, do we have any guidance that we can share at this point in terms of Shopify/hydrogen#729 initialVariantId? I'd like to capture this in the reference topic as well!

Not that I'm aware of at the moment.