docs: Documented the mystery INz bit
docs: Documented the mystery INz bit

This was figured out by a combination of looking at older XPLA3
documents as well as testing against actual hardware.

azonenberg/openfpga,rqou/openfpga,azonenberg/openfpga,rqou/openfpga,azonenberg/openfpga,rqou/openfpga,azonenberg/openfpga,azonenberg/openfpga,rqou/openfpga,rqou/openfpga