Feature request: provide warning if Jetpack Search is active and site has been set to discourage crawlers

#### Is your feature request related to a problem? Please describe.
The root cause of my problem with search filters not displaying in #11116 was that the site had not been completely indexed by Jetpack. This is probably because Jetpack was installed, and Search was activated, when the site had had "Discourage search engines from indexing this site" checked in **Dashboard > Settings > Reading**.

#### Describe the solution you'd like

1. Provide documentation on https://jetpack.com/support/search/ to the effect that Search and Search filters may not work if the site has the "Discourage indexing" box checked, because this prevents Jetpack's indexer from running
2. Provide an error message or other warning at `wp-admin/admin.php?page=jetpack#/traffic` in the "Search" setting box saying that Jetpack Search doesn't work with "Discourage indexing" checked.
3. Fall back to WordPress Core search results when Jetpack Search results cannot be relied upon because of absent or incomplete indexing.

#### Describe alternatives you've considered

We tried using Jetpack with indexing discouraged; it didn't work. Therefore, it seems that documentation and code checks are the best way to proceed.

#### Additional context
This is a follow-up to #11116, and is part of https://github.com/Automattic/jetpack/issues/5780.

Start by reviewing the context in issues #11116 and #5780, then read the Jetpack Search documentation at jetpack.com/support/search/ and the Search setting at wp-admin/admin.php?page=jetpack#/traffic. Done should cover the requested documentation, an appropriate warning for the Reading setting, and a defined fallback when indexing is absent or incomplete.

Written by the indexing model from the issue text.