bpo-44800: Document internal frame naming conventions (GH-32281)

bpo-44800: Document internal frame naming conventions (GH-32281)

The fact interpreter frames were split out from full frame objects
rather than always being part of the eval loop implementation means
that it's tricky to infer the expected naming conventions simply
from looking at the code.

Documenting the de facto conventions in pycore_frame.h means future
readers of the code will have a clear explanation of the rationale
for those conventions (i.e. minimising non-functional code churn).