Speculative Loading setting for authenticated users should include warning if persistent object cache is not present

[westonruter]

Feature Description

In #2097, a new setting was added for Speculative Loading to opt in when a user is logged in:

As noted in the setting's description, this should only be enabled when the server is adequately configured for scaling uncached requests. The easiest way to achieve this is by ensuring that a persistent object cache is enabled. The description should be updated to explicitly mention this and link off to the documentation URL such as is computed in the Site Health test.

Additionally, if wp_using_ext_object_cache() returns false, then an inline warning admin notice should be displayed before the list of radio buttons that advises against enabling Speculative Loading for authenticated requests until a persistent object cache is enabled.