include: sframe: update code comments around SFrame FRE stack offsets

include: sframe: update code comments around SFrame FRE stack offsets

This also amends the incorrect comment:
    offset3 (intrepreted as FP = CFA + offset2)

If RA tracking is enabled,  the offset to recover FP is at the third
index.  The SFrame format (V2) has assumption that if FP is saved on
stack, RA must have been saved as well.  This is true for the currently
supported arch Aarch64.  For AMD64, RA tracking per SFrame FRE is not
necessary.

In future, when extending support for more architectures, this will
likely need to be revisited.

include/
	* sframe.h: Make the comments clearer by enumerating what
	happens per-ABI.