Document `path` behaviour and access restrictions

[KtorZ]

Description

The cache restore strategy behavior seems to include the path somehow, which may come as a surprise when trying to reuse caches across jobs/workflows.

Motivation and Context

Thanks to this comment which saved me some headache; I thought it wouldn't hurt to add some warning for the next person.

Possibly related to: #1561, #1491 and #1426.

How Has This Been Tested?

The behavior was confirmed (v4) when trying to re-use a cache built from a separate workflow job. No cache-hit initially because of a slightly different path value; cache hit as soon as this was fixed on the consumer side.

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation (add or update README or docs)

Checklist:

• [ ] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.