Update the roll forward doc to document no inheritance (dotnet/core-setup#5497)

Update the roll forward doc to document no inheritance (dotnet/core-setup#5497)

The original version of the doc defined that chained framework references would inherit effective roll forward settings (with possible overrides).

But the code never implemented the behavior. The behavior in code is such that each `.runtimeconfig.json` is processed in isolation.

Changing the document to match the code.

Commit migrated from https://github.com/dotnet/core-setup/commit/c638300eb750c6e731495a756a8e35e11e7bd844