Adding NKRO virtual dip-switch, using existing bit in keymap byte.

This takes the last reserved bit there, but doesn't necessitate a revision to the magic number because it doesn't alter byte order.
Add reference to NKRO virtual dip-switch to documentation.

This could be done differently, buy adding a new byte for host controls, but I thought that was a little messy.  (For reference, I did that on a different branch: see 7f520d895834f7a93a04cb8486d82205323215d6.)

I tested this out on my Ergodox.