Docs: extends warning

[M1ke]

Add a warning to extends keyword to note which keys it doesn't extend

Check List

• [x] Tests have been run in packages where changes made if available (n/a)
• [x] Linter has been run for changed code (n/a)
• [x] Tests for the changes have been added if not covered yet (n/a)
• [x] Docs have been added / updated if required (docs only)

Description of Changes Made (if issue reference is not provided)

The extends keyword does not apply refresh_key to the cube that's extended, if the base cube is abstract, instead that cube will use the default refresh. There's no indication of this being a bug but the docs don't clarify it. Checking the code it's unclear if this is deliberate but it feels like this would be a clear bug otherwise so assuming it's intended. This clarifies the behaviour to avoid unnecessary cache refreshes.

[M1ke]