Add a search suggestion warning in the settings

In #42801 (closed) we decided to still allow the search suggestion settings in about:preferences#search. In case the feature might be useful to a user.

However:

1. Currently, our built-in search engines are not configured to handle suggestions. So this is only usable when the user adds a custom search engine which handles suggestions. Which is potentially problematic anyway. #44117.
2. Search suggestions potentially open up another privacy risk by revealing early iterations of a search to the provider, before the user confirms the search with that search engine.

Therefore, I wonder if we should:

1. Disable and clear this setting if no search engine supports suggestions.
2. Add a warning about the privacy risk.

The second could be handled by #44117, in which case we can just handle the first part in this issue.