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

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 .

Open the FIRRTL Rationale page and go to the “Input and Output Module Ports” section linked in the issue. Check the statement about IntegerAttr against the current DenseArray encoding described in the issue, then update the rationale so it is accurate; the work is done when that section no longer describes the old representation.

Written by the indexing model from the issue text.