Add a notice for Instant Results in network mode.

[JakePT]

Description of the Change

Adds a notice to the Instant Results feature when ElasticPress is network activated. The notice links to an article which will explain how to save the search template using WP CLI so that Instant Results works for all sites on the network.

Closes #3138

How to test the Change

1. Network Activate WooCommerce.
2. In ElasticPress > Features there should be a warning message in the Instant Results box.

Changelog Entry

Added - A notice to the Instant Results feature settings when ElasticPress is network activated that warns that Instant Results will not work on all sites without additional steps, with a link to an article that describes the required steps.

Credits

Props @JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.