[FIRRTL] Update rationale mentioning use of IntegerAttr for port directions

[dtzSiFive]

As of #6875, port directions as now encoded with a DenseArray.

In the rationale we mention we still encode with IntegerAttr, this should be updated to reflect what's been learned and changed.

https://circt.llvm.org/docs/Dialects/FIRRTL/RationaleFIRRTL/#input-and-output-module-ports .