Add note about necessity of distributed caching

What does this MR do?

When multiple runners are in use and may pick up jobs with the same tag supplied, the cache isn't shared between them unless a distributed storage is setup.

This adds a note in the good caching practices section to make this more apparent for customers having such a multi-runner-same-tags setup, since the first point under it does not imply it.

Related issues

Customer ticket: https://gitlab.zendesk.com/agent/tickets/236920

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